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goodlookin1
08-29-2015, 8:23 PM
Hey guys,

Finally took out my newly rebuilt M110 styled 308 to the range today. Shot well, but it was new and not yet broken in yet. I lubed it and the action felt very smooth (unlike most AR's before they've been shot.....they feel very abrasive [sandpapery] ). So I was surprised to find that it was shortstroking and not locking back after the last shot.

I made sure I had everything correct in my build: My barrel is a Criterion 20" M110 profile and has a rifle length gas system. For some reason, in spite of being an LR308 based,Criterion opted drill these barrel's ports for the Armalite AR-10 rifle length gas tube instead of the DPMS LR308 rifle length gas tube. It was hard to find, but I did verify that I purchased the correct AR-10 rifle length gas tube.....it is ever so slightly longer. Apparently if you use the LR308 gas tube, short stroking can ensue. Also using a standard rifle length A2 buttstock for 308.....verified it has the correct spring rate.

Not sure what else it could be at this point. My reloads were not "hot loaded", but they were one step down from the max using 4895 and a 175gr AMAX.

Any other ideas for why I might be short stroking? In your opinion, is this something I should be looking into at this point, or does the action need more break-in time?

Thanks.

shafferds
08-29-2015, 8:51 PM
Gas block not aligned properly or gas key issues.

Cadre
08-29-2015, 8:59 PM
carbine or rifle length tube?

goodlookin1
08-29-2015, 9:08 PM
Gas block not aligned properly or gas key issues.

Good Call. I'll have to check those. Pretty sure I aligned the gas block correctly as far as the lateral positioning, but the depth could possibly be off....

As for gas key, what are the signs that I should be looking for? Being loose or gas leaks from areas that are not supposed to be leaking gas? BCG is from Radical Firearms.

carbine or rifle length tube?

Rifle length everything ;)

TygerAR
08-29-2015, 9:12 PM
Hey guys,

I made sure I had everything correct in my build: My barrel is a Criterion 20" M110 profile and has a rifle length gas system. For some reason, in spite of being an LR308 based,Criterion opted drill these barrel's ports for the Armalite AR-10 rifle length gas tube instead of the DPMS LR308 rifle length gas tube. It was hard to find, but I did verify that I purchased the correct AR-10 rifle length gas tube.....it is ever so slightly longer. Apparently if you use the LR308 gas tube, short stroking can ensue.

Thanks.

You already know what is wrong. If the gas tube does not go all the way into the gas key, there will be a loss of gas pressure. I'd try the Armalite tube.

97F1504RAD
08-29-2015, 9:21 PM
You already know what is wrong. If the gas tube does not go all the way into the gas key, there will be a loss of gas pressure. I'd try the Armalite tube.

He said he made sure and purchased the longer tube.

OP what spring and buffer are you using? And I would try a few boxes of factory ammo and see how she runs.

nate76239
08-29-2015, 9:39 PM
I agree about checking for gas leaks look for too much carbon fouling on parts like the gas block/ gas tube junction see this thread i put some pictures of a gas leak that helped somebody else
http://www.calguns.net/calgunforum/showthread.php?t=1099425

baih777
08-29-2015, 9:51 PM
So I was surprised to find that it was shortstroking and not locking back after the last shot.


please clarify. are you saying its shortstroking only after the last round ?? if it is only the last round, that's not shortstroking. if its not feeding every round then it could be shortstroking.

which lower are you using ? which LPK did you use ?

renardsubtil
08-29-2015, 10:38 PM
How many rounds have you shot through it?

If you fire one round, does the bolt lock back?

What's the length of the buffer spring and buffer? What's the buffer weight?

Are you using an adjustable gas block?

I just got over short stroking issues on my 20" rifle build but it may have been a combination of things. I waited to break the rifle in till around the 100 round mark (per a fellow CGer's suggestion), the rifle still had short stroking problems so I changed my buffer spring and buffer out for an Armalite combo and now my rifle will eat any ammo I feed it. But just start with the answering the above because I think these AR10s can vary slightly from build to build

SloChicken
08-29-2015, 10:48 PM
when you align the gas tube, get some feeler gauges and put 0.030" worth and separate the shoulder of the block from the shoulder of the barrel when you align. The rifle/barrels are designed to accommodate a forward cap from a conventional (delta ring) hand guard.
As said earlier, likely suspects are misaligned gas block(if it is an adjustable, open adjuster to allow max gas to enter the tube).
Check to make sure the hand guard/barrel nut is not misaligned and skewing the tube aside. It should have a bit of wiggle room where it goes through the receiver.
Last, and not very likely, make sure it is reaching the gas key completely.
Other than that, it could be:
Too small of a port in the barrel (not very likely)
Too long of a buffer
Too heavy of a buffer
Too short of a buffer tube to accommodate the buffer you are using
Too long of spring for buffer tube/buffer setup
Too heavy rate of spring (also not likely)

So, it could be lots.

But I don't think the lower or LPK has much to do with it (not sure what the poster above is thinking)

My bet is it is a misaligned gas block or tube.
Possibly too long of a buffer for the buffer tube.

ar15barrels
08-29-2015, 10:54 PM
Finally took out my newly rebuilt M110 styled 308 to the range today.

For some reason, in spite of being an LR308 based,Criterion opted drill these barrel's ports for the Armalite AR-10 rifle length gas tube instead of the DPMS LR308 rifle length gas tube.

The reason would most likely be that KAC uses AR-10 length gas tubes on SR25's and M110's and Criterion was following KAC's lead.

The M110 is NOT LR-308 based.

The AR-10 "standard" (if there was one) is typically 308 sized receivers with an angled transition at the buffer tower, 18tpi barrel nut threads, angled magazine well for notched M14 magazines with an AR-15 length gas system which requires longer gas tubes than an AR-15 due to the differences in bolt/barrel extension length.

The KAC "standard" (if there was one) is typically 308 sized receivers with an angled transition at the buffer tower, 16tpi barrel nut threads, straight SR-25 magazine well with an AR-15 length gas system which requires longer gas tubes than an AR-15 due to the differences in bolt/barrel extension length.

The LR-308 "standard" (if there was one) is typically a 308 sized receivers with a radiused transition at the buffer tower, 16tpi barrel nut threads, straight SR-25 magazine well with a shortened gas system to be able to use AR-15 gas tubes.

ar15barrels
08-29-2015, 10:58 PM
Rifle length everything ;)

What's the gas port diameter?

Hoop
08-30-2015, 7:09 AM
Before you go tearing everything apart why don't you tell us what load you were using & what factory ammo you tried...

KillZone45
08-31-2015, 12:27 AM
My guns problem was the buffer and spring. I was using a Slash's buffer and spring and went back to the stock LWRC set up and it ran fine after that. Just some food for thought.

Divehobo
08-31-2015, 1:21 AM
I was having same issue and originally thought a DPMS 10 round mag was bad. I did a lot of reading and research. Everything changed and problem resolved when I installed a JP Silent Captured Spring. Rifle has not had a hiccup since.

major burnout
08-31-2015, 2:26 PM
If you find that the gas system is good, try manually cycling the action 100 times or so. Even though the action feels smooth it still might need a break in.

barron123
08-31-2015, 3:07 PM
I tore my LR-.308 apart three times thinking gas leaks, or buffer/spring issues. Turns out the P-mags I had were the culprit. As long as I run DPMS mags it will run fine.

97F1504RAD
08-31-2015, 3:23 PM
I tore my LR-.308 apart three times thinking gas leaks, or buffer/spring issues. Turns out the P-mags I had were the culprit. As long as I run DPMS mags it will run fine.


What was the problem with the PMAGS?

Hoop
08-31-2015, 3:52 PM
Some of the newer PMAGs have been finicky, I haven't had any problems but have heard of others having issues with the new 10rders.

97F1504RAD
08-31-2015, 4:43 PM
Me neither which is why I ask. Curious to know what issues.

barron123
08-31-2015, 6:49 PM
What was the problem with the PMAGS?

Not sure, but I was getting ftf like crazy! I think the bullets sat a little low because the bolt would strip off the next round and shove it into sides of the chamber. I tried factory ammo and reloads. Switched to dpms mags, and problem solved.

97F1504RAD
08-31-2015, 6:58 PM
Not sure, but I was getting ftf like crazy! I think the bullets sat a little low because the bolt would strip off the next round and shove it into sides of the chamber. I tried factory ammo and reloads. Switched to dpms mags, and problem solved.


Did you contact Magpul with regard to your issue?

goodlookin1
08-31-2015, 7:48 PM
Thanks for the replies everybody. I have a lot to go on here.

I'll dive into it and take a look, but the thing that caught my attention was the magazines: I used to have 10/20 pmags from years before, but I sold them when I sold my 308 to get a longer range type 308. Since then, I have acquired 10 round pmags that are very new and I am left wondering if those could be the culprit?

My rifle never failed to strip and feed a round from the mag, but locking back after the last round was problematic. Is did lock a few times, but failed about 75% of the time.

Reloads I ran were 178gr AMAX at 39.2gr of IMR 4895 seated at 2.80". That's near max load for a "service rifle" while the max for commercial spec is 40.2.

This range session was this rifles first. I ran about 30 rounds through her. Shoots really nice and smooth and was not disappointed. But I definitely want it to lock back after the last round.

Randall, the reason I thought the M110 gas length was strange on this barrel is because the barrel has an LR308 based barrel extension (intended for LR308 builds) while maintaining remnants of the actual M110 specs (AR-10 Armalite based). It's confusing to me from a business standpoint.....it would likely only confuse builders and get bad reviews from buyers who are none the wiser. But I digress.

So my order of operation for investigation is thus:

1) check out potential mag issue
2) check out potential gas block misalignment (depth on barrel)
3) check gas port size
4) check gas key for potential leak
5) check buffer spring and buffer combo and verify correct weight

barron123
08-31-2015, 7:51 PM
Did you contact Magpul with regard to your issue?

No, never thought I would have any recourse with them. I really wish they ran for me. I've had great luck with all their other products.

goodlookin1
08-31-2015, 7:54 PM
By the way, ever had a pattern like the attached picture before? This was not intended! All the shots in the bullseye were taken starting from middle left and moved in a circular clockwise pattern.

I think the scope is either off, the mount wasn't tight enough, or the rings weren't tight enough! Very interesting, right? Shots in the back were pre-zero.

97F1504RAD
08-31-2015, 8:03 PM
No, never thought I would have any recourse with them. I really wish they ran for me. I've had great luck with all their other products.

You never know until you try. They very well replace them for you and want your back to check

ar15barrels
08-31-2015, 10:10 PM
What was the problem with the PMAGS?

New pmags often cause problems in new rifles.
The new mags need to be fired a few dozen times to free up how the cartridges slide out of them.
A new gun needs the same break-in period to free up the bolt and carrier movement.
All the new parts combined will often cause function problems that don't occur with metal mags as the metal mags let the cartridges slide out easier.
Once a gun is well broken-in, you can introduce new pmags to it slowly and it should run them fine after they are broken in.
The problem only seems to pop up with ALL new parts are used.

ar15barrels
08-31-2015, 10:13 PM
Randall, the reason I thought the M110 gas length was strange on this barrel is because the barrel has an LR308 based barrel extension (intended for LR308 builds) while maintaining remnants of the actual M110 specs (AR-10 Armalite based).

AR-10 and LR308 bolts and barrel extensions are identical.
It's only the firing pins that are different.

97F1504RAD
09-01-2015, 6:34 AM
New pmags often cause problems in new rifles.
The new mags need to be fired a few dozen times to free up how the cartridges slide out of them.
A new gun needs the same break-in period to free up the bolt and carrier movement.
All the new parts combined will often cause function problems that don't occur with metal mags as the metal mags let the cartridges slide out easier.
Once a gun is well broken-in, you can introduce new pmags to it slowly and it should run them fine after they are broken in.
The problem only seems to pop up with ALL new parts are used.

Thanks Randall a wealth of information as usual.

Hoop
09-01-2015, 6:44 AM
Reloads I ran were 178gr AMAX at 39.2gr of IMR 4895 seated at 2.80". That's near max load for a "service rifle" while the max for commercial spec is 40.2.


Your rifle is short stroking because your load is so weak it can't break wind

Add another grain and a half and I bet it locks back

41.5-42 in mil brass, 175smk is my load with that powder for my M1A and bolt guns. It's 2550fps most of the time from a 22 or 24, hardly a barn stormer

renardsubtil
09-01-2015, 7:25 AM
This range session was this rifles first. I ran about 30 rounds through her. Shoots really nice and smooth and was not disappointed. But I definitely want it to lock back after the last round.


I suggest maybe shooting at least a 100 rounds first before checking out the buffer item. The other stuff you listed definitely has an immediate impact on the cycling.

liber
09-01-2015, 9:56 AM
New pmags often cause problems in new rifles.
The new mags need to be fired a few dozen times to free up how the cartridges slide out of them.
A new gun needs the same break-in period to free up the bolt and carrier movement.
All the new parts combined will often cause function problems that don't occur with metal mags as the metal mags let the cartridges slide out easier.
Once a gun is well broken-in, you can introduce new pmags to it slowly and it should run them fine after they are broken in.
The problem only seems to pop up with ALL new parts are used.
I don't doubt this, but it seems corny to have to "break in" a mag.

Can't the factory do that?

I've had an occasional feed problem on my LR-308 that sounds similar to barron. I just ordered a couple DPMS mags and want to see if that clears it up. I seat my bullets right at 2.800", they look close in the mag, but do go in.

What happens on mine is occasionally the bolt will catch a round and kick it in cocked and it will jamb, leaving a crease on the shoulder where it catches the center of the feed ramps in the insert. They will shoot fine after I reinsert them into the mag, but it's annoying.

If the DPMS mags help (they are metal I believe) I will look at trying to hone the top around the pmags to see if that helps.

Currently I will get one jamb in about 50 rounds, I'm not sure if it happens on the same mag or not, I should mark the mag next time it happens.

Devilmonkey89
09-01-2015, 10:15 AM
I bought 2 10/25 Magpul M3 mags from audiophil and didn't have an issue running ZQI 7.62x51 out of them.

Hoop
09-01-2015, 11:58 AM
His loads are weak that's the problem...another victim of the terrible data in the Hornady service rifle manual would be my guess...

Devilmonkey89
09-01-2015, 12:22 PM
His loads are weak that's the problem...another victim of the terrible data in the Hornady service rifle manual would be my guess...

What i found weird was that their was primer bulging on the ZQI, hot batch possibly? Don't know... Shot fine.

ar15barrels
09-01-2015, 1:14 PM
I don't doubt this, but it seems corny to have to "break in" a mag.

Can't the factory do that?


At the price they sell them for?
No.

liber
09-01-2015, 2:14 PM
At the price they sell them for?
No.

Maybe that's why the Lancer mags are twice the price... :rolleyes:

liber
09-01-2015, 2:19 PM
His loads are weak that's the problem...another victim of the terrible data in the Hornady service rifle manual would be my guess...

Wouldn't the service rifle section only apply to 7.62x51 cases? Those have a smaller volume in them, where the 308 Win has a slightly larger.

I'm currently using data from Hodgdon site, but I do have a Hornady reloading manual and know there is two sections for both 223 Rem/Service Rifle and 308/Service Rifle.

Or are you saying that the numbers are just plain wrong in the Hornady manual?

I will double check my numbers as I'm going to be loading some IMR 8208 XBR for the first time.

Noonanda
09-02-2015, 5:56 AM
Wouldn't the service rifle section only apply to 7.62x51 cases? Those have a smaller volume in them, where the 308 Win has a slightly larger.

I'm currently using data from Hodgdon site, but I do have a Hornady reloading manual and know there is two sections for both 223 Rem/Service Rifle and 308/Service Rifle.

Or are you saying that the numbers are just plain wrong in the Hornady manual?

I will double check my numbers as I'm going to be loading some IMR 8208 XBR for the first time.

If you compare older reloading manuals, some of the powder or component manufacturers have progressively weakened some of their load data to cover their own butts.
Sounds like Hornady may have, but by Hodgdons load data you are under their min load.
Hodgdon data can be viewed here http://www.hodgdonreloading.com/data/rifle
180 Gr bullet (since they dont list 178, they do have 175 but just as an example)

Bullet Weight 180 GR. SPR SP
Powder IMR 4895
Bullet Diam. .308"
C.O.L. 2.800"
Starting Loads 40.5Grs.
Vel. (ft/s) 2,439
Pressure 43,800 PSI
Maximum Loads 44.7C Grs.
Vel. (ft/s) 2,674
Pressure 58,700 PSI

using 175 Gr load data you are almost 2 grains under minimum (41.0 Gr)

liber
09-02-2015, 9:12 AM
If you compare older reloading manuals, some of the powder or component manufacturers have progressively weakened some of their load data to cover their own butts.
Sounds like Hornady may have, but by Hodgdons load data you are under their min load.
Hodgdon data can be viewed here http://www.hodgdonreloading.com/data/rifle
180 Gr bullet (since they dont list 178, they do have 175 but just as an example)

I have both Speers and Hornady, as well as an older Lyman but I don't use the Lyman as it doesn't have any recent powders, like it doesn't even list Varget.

I do see that the Hodgdon site has slightly different numbers. On 168 BTHP it's almost the same, but not on 175. BTW, that is one of the things I don't care for in the Hornady manual, they use Hornady size bullets and/or have a few grain spread on the numbers they give where they will say 165-168 grain...Speer, OTOH, doesn't list 175 and only 180.

That is primarily why I've been using the Hodgdon site, it lets you select the bullet specific.

using 175 Gr load data you are almost 2 grains under minimum (41.0 Gr)

Yeah, Hoop sent me a PM and I was just looking at that data, and honestly hadn't checked that out with 175 before. They are low, for certain. That's kind of odd as for 168 they are just about the same as the Hodgdon site. I do load mostly 168 SMKs, so far.

EDIT: getting back to the OPs problem, would this EVEN be an issue with 2300 fps? Should still have enough to cycle the bolt I would think...I know I have fired 2300 fps loads in my rifle and it wacks those cases out pretty strong. I'm guessing on the 2300 as I haven't cron'd those loads, the only loads I've cron'd were 2400-2500 fps. I'm just going by less powder being less fps. I'm also not sure what type of loads the OP was shooting. Just tossing some Qs out there.

liber
09-06-2015, 3:38 PM
At the price they sell them for?
No.

Randall,

I disassembled a 308 pmag today:

https://dl.dropboxusercontent.com/u/18576273/pmag-308-disassembled.jpg

Now in looking closely at it, there only seem like 2 areas that could cause a problem.

The rear/middle curved lip where the cartridge feeds out:

https://dl.dropboxusercontent.com/u/18576273/pmag-308-rear.jpg

And the front lip where the cartridge feeds out:

https://dl.dropboxusercontent.com/u/18576273/pmag-308-front-lip.jpg

It seems most likely if a pmag needed to be broken in, that it would be the front lip, as it seems like it could have a tendency to catch the neck rim as the cartridge slides out. I was just testing that and it seems that could be a problem.

I'm going to take a small dremel sanding wheel and put a radius on there, or even just take the front lip down some so that wouldn't be an issue.

I don't see anything else that would make a difference, the rest of the pmap operates very smoothly, just a square groove along the rear which the floor plate slides smoothly in, and a v groove in the front which the floor plate slides smoothly in.

I was just curious when you said pmags need to be broken in, is there one area specific that you were referring to?

ar15barrels
09-06-2015, 5:56 PM
I was just curious when you said pmags need to be broken in, is there one area specific that you were referring to?

Nope.
Just run a couple hundred rounds through the gun and each of the mags before you expect them to be reliable.

liber
09-06-2015, 6:36 PM
Nope.
Just run a couple hundred rounds through the gun and each of the mags before you expect them to be reliable.

I've got almost that much, but not quite, through each of them. I currently have about 600 rounds through it, possibly a tad more. So that's about 150 through each mag (I have 4 mags).

Maybe I should reword my question differently to you. What is the symptom that a non-broken-in mag exhibits, in your experience? Is it an FTF? And if so, how does it typically fail? Thanks for any insight. 2 DPMS mags also on the way, but evidently on the slow boat from MO, but are expected to arrive on Wed. I won't be able to get to the range until the 20th, so no worries, but trying to do what I can before hand.

ar15barrels
09-06-2015, 7:16 PM
What is the symptom that a non-broken-in mag exhibits, in your experience?

FTF in a new gun.
Usually with an operator who is pulling the charging handle back and letting it fly forward on a gun with insufficient/no lube.

CalUSMCvet
09-07-2015, 7:34 PM
Maybe wrong buffer?

the_duck
09-08-2015, 2:01 PM
I had a bad 10/20 Pmag from Jestice that would FTF/FTE...after I took that out of the rotation, I was able to shoot 100 PMC Bronze no problem out of my LR-308.

Figure the rivet job wasn't that great and probably messed with the spring. Will instead of a mag block instead of a rivet, should solve my problems for this mag but I won't get out for another few weeks so I can't be sure.

Can you try out an adjustable gas block? I use my calipers when I place a new gas block on since they don't always butt up against the shoulder, but most of the time they do.

liber
09-20-2015, 7:16 PM
FTF in a new gun.
Usually with an operator who is pulling the charging handle back and letting it fly forward on a gun with insufficient/no lube.

DPMS mags fixed my problem. So, my FTF is definitely a pmag issue.

I didn't get time to run rounds through the pmag I modified, but will try to do more testing soon.

I ran through about 200 rounds through the 2 DPMS mags I purchased with not one FTF.

ar15barrels
09-20-2015, 8:18 PM
I ran through about 200 rounds through the 2 DPMS mags I purchased with not one FTF.


Now try introducing pmags to the gun...

liber
09-20-2015, 8:32 PM
Now try introducing pmags to the gun...

I will do that soon. Unfortunately the RSO had to get home today, so we needed to pack up early, before I could try them. I have 3 unmodified pmags and one that I modified slightly.

I will test with non-modified and modified.

Hopefully I can isolate the problem to understand what exactly is causing it.

FWIW, the DPMS mags were not without a couple issues of their own. At first I couldn't get the mag release to work, it seemed that it was catching and not being able to push far enough with the bullet button. Then I was having problems with both of them not catching, and was fiddling with the bullet button some...finally I could get them to work pretty good. Not one FTF though. The pmags have never given me a problem with catch/release, just the FTF. I will look at this before going back to the range, both need a fairly good jolt to get the catch active.