RCBS Precision Mic - Confused About Shoulder Bump

[HK Dave]

I've been playing around with finding the proper shoulder bumps on my brass and came across a few things that I was hoping someone more experienced could shed some light on.

Here's what I've learned thusfar... and this is all with regards to full length sizing.

Bolt Action Rifle: You want about .002 shoulder bump
Gas Gun: You want .003 to .004 shoulder bump

I have 3 rifles chambered in 308win that I get the following shoulder reading after fire forming my brass. I check this using the RCBS Precision Mic.

Bolt Action #1 with Bartlein Barrel: -1
Bolt Action #2 with Factory Rem 700 Barrel: +3
Gas Gun #1 Larue OBR: -3

So according to these figures, if I understand correctly, the Larue has the tightest chamber and the Factory Rem 700 has the loosest.

The vast majority of my factory match ammo has a factory shoulder bump of -4.

Here's where I get confused....

#1 With the Gas Gun, if I bump it back .003 to .004, I end up with -6 to -7 on the RCBS precision mic which takes me beyond the shoulder bump of factory ammo. That doesn't seem right to me. Should I just bump the brass to match factory ammo of -4 and call it a day?

#2 A lot of my NON Match factory ammo has a shoulder bump of -1 yet it shoots just fine in the Larue. How is this happening? How is it that it fits?

#3 Now on a different subject, but one that has me worried about pressure spikes... when I load a re-loaded round (175 smk to 2.820) into the Bolt Action with Bartlein Barrel or the Larue OBR and don't fire, when I remove the round, the bullets seem to have some marks... what are they? Couldn't be that the bullets are touching the lands right? (On the bolt action, I do notice chambering leaves a small mark on the primer)

Thanks folks for your help and advice. Just trying to keep this sport safe. I like my fingers and stuff!

[buffybuster]

I would double check the shoulder measurements. It can take 2-3 firings of the same piece of brass before it fully blows out the chamber dimensions. New brass often has enough springback to measure a few thousandths short after initial firing.

But presuming your measurements are accurate; do you want a single die setup for your three rifles or are you willing to segregate brass and have at least two different setups?

A couple thousandths of tight headspace is not going to stop the round from chambering, especially with the caming power of a bolt action. The greater concern can be casehead expansion, which can stick the round in the chamber. Based upon casehead measurements, you may have to segregate your brass.

New virgin ammunition is going to be at or below minimum headspace, so it will fit and function in every spec. chamber. When you're reloading you want to full-length resize only the amount necessary for your chamber, this will extend brass life and possibly improve accuracy.

Regarding your question 3: Who chambered your Bartlein barrel and what reamer did they use? If it has the short throat then the bullets could be lightly jammed into the lands. With the semiauto, the marks on the bullets are mostly likely from the feedramp, but again since I don't know the chamber/throat dimensions it hard to be certain. What kind of small mark on the primer? If it is a circle, that is the FP hole rubbing on the primer, the result of high primers. Primers are suppose to be seated a couple thousandths below the headstamp. If you hold a straight edge across the headstamp, you should be able to see a thin sliver of light at the primer, when held up to a strong light.

[FLIGHT762]

I use a Hornady bump gauge and I will give you some of my numbers, which will be a little different than yours, but I will have a SAAMI Minimum which I will call "0". My Hornady .400" gauge measures 1.611" on a 1.630"- .308 Winchester SAAMI "GO" headspace gauge, this will be my "0".

My custom 700 VSF Hart barreled bolt gun chambered in 308 Win. has fired cases that measure 1.613" (+.002" from SAAMI Minimum). I bump these cases only -.001" to 1.612".

My Knight's SR-25 in 308 Win. fired cases measure 1.613" (+.002" from SAAMI Minimum). I bump these cases also to -.001" to 1.612". This auto has a tight chamber and cases bumped just slightly back, chamber effortlessly.

My Remington Custom Shop 40 XBKS with a "7.62 NATO" marked match barrel fired cases measure 1.615-16" (+ .004"-.005" from SAAMI Minimum).

I don't have my notes with me right now, but I have measured quite a number of factory and Military ammo with the bump gauge. I'm pretty certain that most of this Factory ammo measured from -.001" to -.005" from the SAAMI Minimum 1.630" gauge (under 1.611" on my Hornady bump gauge).

As far as question #3 on your bullets having marks on them. What kind of marks?

Bullets and cases will get longitudinal scratches on them from feeding into the chamber. The marks you need to be aware are the tic marks around the ogive of the bullet, which would be your lands. On my Custom Hart barreled 700 VSF, that chamber has a very short throat and when it was new, it would jam the lands with a 175 Sierra OTM @ a length over 1.790".

Now in my Remington 40XBKS, it has the regular Remington long throat.

The SR-25, I don't load over magazine length for it.

You Bartlein barreled gun could have a short throat. The only way to know is to actually measure your bullet lengths to the lands.

Now speaking about shoulder bump in the bolt guns, In my tight Hart barreled gun which fired cases measure 1.613", I can actually chamber ammo that has been F/L sized to 1.615", which gives a light crush fit. I tested this one day using dummy reloads which had been fired in my 40XBKS.

I F/L sized the cases from the 40X , but bumped them only to 1.615". Now, I did have to F/L size the cases because a fired case directly from the 40 X would not chamber in the Hart barrel, but a F/L sizing with the 1.615" shoulder bump cases would crush fit into the Hart barrel.

I'm retired now, so I have time to do all these experiments I never had time to do.

[HK Dave]

I would double check the shoulder measurements. It can take 2-3 firings of the same piece of brass before it fully blows out the chamber dimensions. New brass often has enough springback to measure a few thousandths short after initial firing.

But presuming your measurements are accurate; do you want a single die setup for your three rifles or are you willing to segregate brass and have at least two different setups?

A couple thousandths of tight headspace is not going to stop the round from chambering, especially with the caming power of a bolt action. The greater concern can be casehead expansion, which can stick the round in the chamber. Based upon casehead measurements, you may have to segregate your brass.

New virgin ammunition is going to be at or below minimum headspace, so it will fit and function in every spec. chamber. When you're reloading you want to full-length resize only the amount necessary for your chamber, this will extend brass life and possibly improve accuracy.

Regarding your question 3: Who chambered your Bartlein barrel and what reamer did they use? If it has the short throat then the bullets could be lightly jammed into the lands. With the semiauto, the marks on the bullets are mostly likely from the feedramp, but again since I don't know the chamber/throat dimensions it hard to be certain. What kind of small mark on the primer? If it is a circle, that is the FP hole rubbing on the primer, the result of high primers. Primers are suppose to be seated a couple thousandths below the headstamp. If you hold a straight edge across the headstamp, you should be able to see a thin sliver of light at the primer, when held up to a strong light.

I've segregated brass for each rifle. I segregate by rifle, then by finished weight lots.

The 308 with Bartlein was chambered by GAP. The marks on the GAP chambered rifle look like 5 or 6 marks... kind of like lands. hah What I'm wondering is... could it be that the chamber was cut so tight that the bullet would be pushed up against the lands loaded to magazine length of 2.820"? If so, I should probably push the bullets in to 2.800" for safety sake right? Or I wonder if I'm safe since I'm pushing 175 smk with 43.5 gr of Varget with Rem 9 1/2 primers.

The marks on the primer are indeed a circle.

In regards to the Larue OBR, the mark looks more like a scratch mark... sharp scratch mark. My guess is it happens when the bullet comes in contact with the feed ramps.

[HK Dave]

I use a Hornady bump gauge and I will give you some of my numbers, which will be a little different than yours, but I will have a SAAMI Minimum which I will call "0". My Hornady .400" gauge measures 1.611" on a 1.630"- .308 Winchester SAAMI "GO" headspace gauge, this will be my "0".

My custom 700 VSF Hart barreled bolt gun chambered in 308 Win. has fired cases that measure 1.613" (+.002" from SAAMI Minimum). I bump these cases only -.001" to 1.612".

My Knight's SR-25 in 308 Win. fired cases measure 1.613" (+.002" from SAAMI Minimum). I bump these cases also to -.001" to 1.612". This auto has a tight chamber and cases bumped just slightly back, chamber effortlessly.

My Remington Custom Shop 40 XBKS with a "7.62 NATO" marked match barrel fired cases measure 1.615-16" (+ .004"-.005" from SAAMI Minimum).

I don't have my notes with me right now, but I have measured quite a number of factory and Military ammo with the bump gauge. I'm pretty certain that most of this Factory ammo measured from -.001" to -.005" from the SAAMI Minimum 1.630" gauge (under 1.611" on my Hornady bump gauge).

As far as question #3 on your bullets having marks on them. What kind of marks?

Bullets and cases will get longitudinal scratches on them from feeding into the chamber. The marks you need to be aware are the tic marks around the ogive of the bullet, which would be your lands. On my Custom Hart barreled 700 VSF, that chamber has a very short throat and when it was new, it would jam the lands with a 175 Sierra OTM @ a length over 1.790".

Now in my Remington 40XBKS, it has the regular Remington long throat.

The SR-25, I don't load over magazine length for it.

You Bartlein barreled gun could have a short throat. The only way to know is to actually measure your bullet lengths to the lands.

Now speaking about shoulder bump in the bolt guns, In my tight Hart barreled gun which fired cases measure 1.613", I can actually chamber ammo that has been F/L sized to 1.615", which gives a light crush fit. I tested this one day using dummy reloads which had been fired in my 40XBKS.

I F/L sized the cases from the 40X , but bumped them only to 1.615". Now, I did have to F/L size the cases because a fired case directly from the 40 X would not chamber in the Hart barrel, but a F/L sizing with the 1.615" shoulder bump cases would crush fit into the Hart barrel.

I'm retired now, so I have time to do all these experiments I never had time to do.

From what I'm reading here, it sounds like it's similar to my semi-auto. I find it strange that if the fire formed brass is -3, that the rifle has no problem loading factory ammo that has a -1 shoulder. This makes me think of what buffy said... that fire forming to chamber dimensions can take multiple firings.

I wonder if perhaps because of the XTRAXN fluted chamber in the OBR, brass doesn't flow as far forward, and instead flows into the flutes? This could explain why the gas gun can easily chamber factory ammo with a -1 shoulder.

I really appreciate talking shop with you guys, I'm learning so much about this process.

[FLIGHT762]

The 308 semi autos have a lot of energy with the bolt/carrier flying forward. It actually has enough momentum to bump the shoulder back a bit.

Load a couple of dummy rounds up. Check the bump before chambering and then chamber the dummy and then check the bump after extracting it. It can bump the shoulder back a little.

[HK Dave]

That's a good idea. Agreed.. that 308 bolt feels like it could cut my pinky off. :P

[buffybuster]

I've segregated brass for each rifle. I segregate by rifle, then by finished weight lots.

The 308 with Bartlein was chambered by GAP. The marks on the GAP chambered rifle look like 5 or 6 marks... kind of like lands. hah What I'm wondering is... could it be that the chamber was cut so tight that the bullet would be pushed up against the lands loaded to magazine length of 2.820"? If so, I should probably push the bullets in to 2.800" for safety sake right? Or I wonder if I'm safe since I'm pushing 175 smk with 43.5 gr of Varget with Rem 9 1/2 primers.

The marks on the primer are indeed a circle.

In regards to the Larue OBR, the mark looks more like a scratch mark... sharp scratch mark. My guess is it happens when the bullet comes in contact with the feed ramps.

By your description, it appears like your rounds are lightly jammed into the lands. If you are using Winchester brass with that load, you're still within the safe pressure zone. If you're running Federal or LC brass (heavier, lower capacity), you could be running higher pressures. What you can do to find out your throat length is take a FIREFORMED case, NECK SIZE it, notch the neck so it will hold a bullet but not too firmly. Load a bullet long, carefully chamber the dummy round and carefully take it out and measure the ogive length. Do this a few times to get a consistent measurement. That is the distance to the lands.

How are you seating your primers?

[HK Dave]

The 308 semi autos have a lot of energy with the bolt/carrier flying forward. It actually has enough momentum to bump the shoulder back a bit.

Load a couple of dummy rounds up. Check the bump before chambering and then chamber the dummy and then check the bump after extracting it. It can bump the shoulder back a little.

Yes! The mystery of the gas gun is solved. I chambered a factory round that has a -1 shoulder and let the OBR slam the round into battery. Took it back out and remeasured. It's not a -4. This explains much.

[HK Dave]

By your description, it appears like your rounds are lightly jammed into the lands. If you are using Winchester brass with that load, you're still within the safe pressure zone. If you're running Federal or LC brass (heavier, lower capacity), you could be running higher pressures. What you can do to find out your throat length is take a FIREFORMED case, NECK SIZE it, notch the neck so it will hold a bullet but not too firmly. Load a bullet long, carefully chamber the dummy round and carefully take it out and measure the ogive length. Do this a few times to get a consistent measurement. That is the distance to the lands.

How are you seating your primers?

I took a round loaded to 2.820" and chambered it into the GAP 308 and the marks look like 5-6 separate marks... looks like where the lands would be. Took a round loaded to 2.800" and same marks are there, but less pronounced.

I'm running 175 smk, 43.5 gr Varget, Remington 9 1/2 primer with Federal Commercial Brass.

I originally worked up my load at 2.800" OAL and didn't see pressure signs until I hit 44.5 to 45 gr.

Sadly, I loaded 300 rounds of 43.5 gr to 2.820". I wonder if it'll be ok to shoot. Maybe I should back them down to 2.800" since it looks like the bullet is kissing the lands even at that length.

Priming i do with the RCBS hand primer. I'll put a little more squeeze into it.

[LynnJr]

H K Dave
First off most accuracy shooters fireform there brass 2 times before bumping the shoulder and the reason is obvious-You can't bump the shoulder if it is too short in the first place as there is nothing there to bump.
You also don't need any fancy tools to measure shoulder bump but since you bought on you might as well use it.

For your bolt action rifles here is an easy way to do it.
Take one case out to the range and fire it.When you get home knock the primer out put a new one in add powder and a bullet and head back to the range.You don't full length re-size your brass you partially re-size your brass just so the neck will hold onto the bullet.To do that back the die off the ram 1/8 inch.

You repeat this until the bolt gets a little bit hard to open or what is commonly referred to as bolt click.When you lift the handle on a typical 2-3 lug action bolt gun nothing happens until the handle is almost all the way up.At this point the handle makes contact with the extraction cam which actually moves the bolt itself rearward.Up until that point the bolt does not move rearward.A typical extraction cam will move the bolt rearward less than 1/4 inch and the shooter does all the rest.

Now you measure its length from the base to the shoulder and subtract the 0.001 or 0.002 for your shoulder bump.Write it down in your die box and make sure you put down which rifle it is for as they will all vary +/-0.005

That is all you need to know about setting the shoulder bump.It will be specific to each of your rifles and it is a complete waste of money trying to synchronize them.

In a nutshell each time you reload your full length die will be set to match each rifle and that will not change until you re-barrel the gun no matter whose brass you use.

Edit: I just re-read your post and forgot to answer your last question.
The lands in your barrels throat are not abrupt or 90 degrees to the bullet.Most target rifles use a 3 degree included angle or 1.5 degrees per side.As you can see 1.5 degrees is a very shallow angle and your bullets will get land marks way before they are at the full jam length.If your magazine will allow the 2.820 cartridge over all length there is no reason not to shoot them if they are accurate.

If they are not accurate now would be a good time to find your ideal seating depth as you already have the rounds ready.
Good Luck

[stilly]

you mean I gotta go through this when I setup my precision mic and dies for my .223 and .308?

damn...