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View Full Version : bullet drift from cold to hot barrel


Slash2
07-04-2009, 2:51 PM
I have a 20" barrel .308 hunting rifle that the point of impact will drift 7" down and 2" to the left as the barrel goes from cold to hot over the course of about 10 rounds.:eek:

Once the barrel is hot it keeps a nice group of around 1.5" at 100 yards.

The gun isn't new but the bore is in good shape. I get the same results with different scopes, with open sights, factory and hand loaded ammo and even with cleaning the bore between shots.

I realize that with a hunting gun the first couple of shots out of a cold barrel are all that matter, but this excessive. I'd like to have a better idea of what's going on and what steps might be available to correct it.

postal
07-04-2009, 3:33 PM
The very first step is usually to bed the action into the stock.

However a lot of wood stocks move a good deal so it's best to have a synthetic/polymer/fiberglass.

Next big jump up in price and benefit is a new after market match grade barrel.

emilio
07-04-2009, 3:56 PM
The very first step is usually to bed the action into the stock.

+1, but not just bed, make sure the barrel is floated.

really wild swings like that just reek of barrel contact (if the barrel is not obviously bent, bulged, or cracked), though it does seem excessive. do the "dollar test": take a nice crisp Federal note, run it under the barrel like you were going to polish it, and slide the dollar back to see how far you can go before the it drags or hits something. a free-floated barrel will let a dollar slide all the way to the receiver ring. i know that base model Rem. 700s with the poly stock have two or three little studs that contact the barrel to stabilize the forend

if this is the case, the answer would be to free float the barrel (bedding is nice, but they are exclusive). if the stock cannot take free floating, for instance a weak forend that is stabilized on the barrel, then reinforcement or a new stock may be in order.

- emilio

RaymondMillbrae
07-04-2009, 5:07 PM
Sounds like the barrel has given up the ghost.

I deduce that the barrel has wore out, and widened-up a bit. (It has a larger inside diameter).

So when the first few shots are fired, the rounds are sloppy. But as the barrel heats up, the metal expands and reduces the inside diameter of the barrel - hence giving you tighter groups.

I will also ask if you are doing DEEP, DEEP, cleaning of your barrel. (Like with JB Cleaner).

Why?

Because this is a very harsh and abrasive cleaner that takes everything down to the bare metal - which should not be done too often. (Actually, every blue moon in the life of the barrel). You may have "scrubbed away" the metal and enlarged the inside diameter of the bore if cleaned too many times.

Just my initial thoughts on the matter.

In Christ: Raymond

PS: If you have an endoscope (or some type of scope), take a gander into your barrel. Look at the lands and grooves of your barrel. The grooves should have, sharp, 90-degree angles, and be nice and deep. If they are rounded-out, or kind of shallow...your barrel is worn out.

phish
07-04-2009, 8:20 PM
But as the barrel heats up, the metal expands and reduces the inside diameter of the barrel - hence giving you tighter groups.


I lol'd

Timberwolf
07-04-2009, 8:33 PM
I am assuming this is a pencil barreled sporting rifle. That said what is happening is simple the barrel is heating, expanding and rubbing against a high spot in the stock. Free floating may or may not be your solution as some sporter barrels like a little pressure, some don't. Since this is a huntng rilfe your accuracy for your first three shots is all that matters as if your have to expend enough rounds at one animal to heat your barrel sufficient for it to change it POI you'd better hang it up anyway. Long story short, shoot three rounds from a cold barrel and see how it groups and zero from there.

Prc329
07-04-2009, 8:44 PM
Is it a savage? Some are known to do that due to the barrel threads not being staight. Ryan (rksimple) can tell you all about it.

CSACANNONEER
07-04-2009, 8:54 PM
Sounds like the barrel has given up the ghost.

I deduce that the barrel has wore out, and widened-up a bit. (It has a larger inside diameter).


IMHO this is not very likely. The barrel still shoots nice tight groups but the POI is wandering a bit while the barrel is heating up. This is normal and every barrel will show different amounts of change during the heating up proccess.


So when the first few shots are fired, the rounds are sloppy. But as the barrel heats up, the metal expands and reduces the inside diameter of the barrel - hence giving you tighter groups.


I've never heard that one before. Thanks, I'll use it next time I need to make someone laugh!


I will also ask if you are doing DEEP, DEEP, cleaning of your barrel. (Like with JB Cleaner).

Why?

Because this is a very harsh and abrasive cleaner that takes everything down to the bare metal - which should not be done too often. (Actually, every blue moon in the life of the barrel). You may have "scrubbed away" the metal and enlarged the inside diameter of the bore if cleaned too many times.


Funny, I have watched Lee Rasmussen (yea, this guy: http://fcsa.org/wwwroot/visitors/worldrecords.php) clean his rifles with the RED JB paste both during and after every match I've that I've had the pleasure of shooting with him at. His barrels seem to do OK and last longer than many other 50 barrels when he uses the EXTRA COURSE JB paste.

Can you please reference where you are getting your info from. It would be interesting to find out.

Trapper
07-04-2009, 9:03 PM
I'd buy a good quality synthetic stock or glass bed and free float the factory stock. What is the make and model of your rifle? Is the barrel free floated?

postal
07-04-2009, 11:41 PM
The very first step is usually to bed the action into the stock.



+1, but not just bed, make sure the barrel is floated.


- emilio

Ah- yeah. Absolutely correct. I forgot and left that out. Good call.

phish
07-04-2009, 11:45 PM
Another possible cause is that the barrel was bent during manufacture and was subsequently straightened before going out the door. Heating the barrel through firing will cause it to begin drifting back to its bent status as the stress is relieved from the straightening process.

In any case, several diy workarounds have already been mentioned, give those a try.

Slash2
07-05-2009, 6:09 AM
Good info guys!

Here's a little more background info. The gun is a Savage 99E .308 lever action. Barrels for it are very hard to find unless custom made and frankly the gun is not worth that expense. The rifling is good, a bullet placed in the muzzle contacts the same place as on a new barrel.
It is a copper mine when fired so I did give it a deep cleaning with JB paste to bare metal.
The barrel did contact the fore end stock at the muzzle end of the grip, I put a sleeve in the fore end screw hole so the the only contact with the barrel was with the screw, the wood doesn't touch now.
The big non-original aspect of the gun is that it has been free bored, by alot. It has exactly .125" bullet jump before contacting the lands, and that's with me seating a 180 gr round nose bullet to my maximum magazine length, 2.35".
My last range visit I fired one shot and cleaned the barrel and let it cool, then repeated that 10 times. The POI continued to migrate to 7" down and 2" to the left. The last three shots grouped within an inch.

tankerman
07-05-2009, 6:21 AM
make sure the foregrip bolt is not too tight.

huckberry668
07-05-2009, 9:15 AM
Don't use JB or anything abrasive!! You're just wearing out the rifling faster. You won't see your groups getting smaller. Ask me how i know.

Fjold
07-05-2009, 11:42 AM
Go talk to the guys at Savageshooters.com. They can help you narrow down your options quickly.

Slash2
07-05-2009, 3:21 PM
Go talk to the guys at Savageshooters.com. They can help you narrow down your options quickly.

Savageshooters.com looks like it might be the place to go for answers, I'm surprised I hadn't found it already! Thanks.

hybridatsun350
07-05-2009, 4:06 PM
Sounds like the barrel has given up the ghost.

I deduce that the barrel has wore out, and widened-up a bit. (It has a larger inside diameter).

So when the first few shots are fired, the rounds are sloppy. But as the barrel heats up, the metal expands and reduces the inside diameter of the barrel - hence giving you tighter groups.

I will also ask if you are doing DEEP, DEEP, cleaning of your barrel. (Like with JB Cleaner).

Why?

Because this is a very harsh and abrasive cleaner that takes everything down to the bare metal - which should not be done too often. (Actually, every blue moon in the life of the barrel). You may have "scrubbed away" the metal and enlarged the inside diameter of the bore if cleaned too many times.

Just my initial thoughts on the matter.

In Christ: Raymond

PS: If you have an endoscope (or some type of scope), take a gander into your barrel. Look at the lands and grooves of your barrel. The grooves should have, sharp, 90-degree angles, and be nice and deep. If they are rounded-out, or kind of shallow...your barrel is worn out.

Dude! Quit making **** up! lol :p

SteelRain
07-05-2009, 8:26 PM
Ok. The sky is the limit for reasons that could cause this problem. First of all. You mentioned that once the barrel heats up the POI (point of impact) changes. Is it consistent? Is it vertical or horizontal stringing? or is it sporadic?

In most cases like you said your cold shot may be consistent with a bent barrel. If your barrel is bent, when heated its molecular structure attempts to return to its orginal state as it has some what of a memory so to speak.

Not all rifles need to be free floated. in fact, some barrels like to be pressure bed. this might help.

PM me with more specific symptoms and i might be able to help you out.

Steel Rain Gunsmithing

Ahhnother8
07-05-2009, 8:52 PM
Most likely internal stresses in the steel. Hammer forged or less likely button rifled barrels, will have stresses. Heat any metal and it WILL move. Even if left to cool to the touch on the exterior, the interior may still retain enough heat to cause POI changes. There may not be a good solution, except to not miss the critter on the first shot?????

emilio
07-06-2009, 9:40 AM
yeah, lever action TOTALLY changes the game. there's probably a handful of barrel contact points, if not continuous. i don't know much about lever accuracy, but i'd say look for loose barrel bands or ones with uneven pressure. i wouldn't think free-bore would consistently change POI like that, but i haven't experimented much with free-bore in high power cartridges.

also, iron sights or scope? mounts? maybe the gun is shooting straight, but the sights are drifting with heat.

that Savage forum is probably your answer!
- emilio