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Gunsmithing 16 inch 308 keyhole problem?

Sniperhandle

Sniperhandle

Sergeant
Full Member
Minuteman
Mar 18, 2011
197
2
61
Southern Utah
#1
I would like to ask the more knowledgeable smiths on this forum a question about a 308 rifle I recently built with a 1 in 12 barrel I shortened to 16 inches. 168 grain bullets are keyholeing on me. I did not take this into consideration when shortening it, I just wanted a handy rifle. I have not tried lighter bullets yet. Is this twist rate too slow now because of the length? I have an ar10 with a 16 inch barrel that shoots fine.
 
#7
Re: 16 inch 308 keyhole problem?

I set my compound at 11 degrees and cut from the center out like I always do, never had a problem with the other barrels I have done, I am just a hobbyist though and have no formal training.
 
#8
Re: 16 inch 308 keyhole problem?

What are the specifics on your ammo? (factory?, reloads?, If reloads, what's the data on your load?, etc., etc.)

A 1:12 twist barrel, even in a 16" barrel, SHOULD properly stabilize a 168gr bullet.

As for your AR-10...assuming you mean an Armalite...my guess is that it has a 1:11.25 twist, so be cautious comparing apples to oranges.
 
#10
Re: 16 inch 308 keyhole problem?

A guy had a keyholing problem a while ago on another forum with his remington in 308. He racked his brain along with everyone else who offered up a possible reason for it. In the end it turned out that his scope was so far out that the bullets were actually hitting the ground in front of the target and skipping to the target.
 
#12
Re: 16 inch 308 keyhole problem?

I am using my own loads, the barrel is a parker hale m82 cadet sniper rifle barrel. 42 gns varget 168 smk, I have had very good results using these barrels, It may be a bad barrel. It looks real good inside, but I do not own a borescope.
 
#15
Re: 16 inch 308 keyhole problem?

I have a 16" 700 with a factory tube. No problems with keyholing for me. Accuracy is good to great with the same loads I use in my "regular" length rifles.

Might try launching them a bit faster.
 
#16
Re: 16 inch 308 keyhole problem?

Check the twist rate first. It may be slower than labeled. Probably the speed you are getting is at the ragged edge of stability for that bullet. A shorter bullet in that barrel may help. The Sierra 165 HPBT is extremely accurate and a bit shorter. A flat base 165 may be the answer. Cheaper than a new barrel. JMHO
 
#17
Re: 16 inch 308 keyhole problem?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sniperhandle</div><div class="ubbcode-body">I would still like to know if building a 308 rifle with 16 inch barrel you would need faster twist rate than 1/12 ? </div></div>

All things being equal...NO, you shouldn't need a faster twist (then again...I haven't built a .308 with a 1:12 twist in 20yrs because I don't like the lighter pills and a 1:11.25 or 1:10 twist barrel is superior for shooting a wider, more useful range of bullets for my purposes)!

Try running some factory ammo (like FGMM 168gr, BH 168gr, etc.) through the gun and see if your problems disappear. If they do, then you know it is an ammo problem with your ammo in that rifle. If the problems remain, it is likely a barrel problem (either with it stabilizing 168gr loads, or a flaw that will not allow the barrel to stabilize anything...you can confirm with shooting a proven lighter weight bullet load from the rifle).
 
#20
Re: 16 inch 308 keyhole problem?

12 twist is fine, works for the xp-100's shooting 168's. check the end of the barrel again, maybe you did leave a burr that you don't notice. rough up a q-tip and drag it from the muzzle out, if it leaves a string behind you've got a klingon. q tip also works well to leave in muzzle of barrel and shine a light to get a better detailed look see. Not tough to fix w/o putting back in lathe. Also if you have chronograph, get some shots over that for more data.
 
#23
Re: 16 inch 308 keyhole problem?

WHY did you NOT tell us before you had a brake on
smirk.gif

Take the damn thing off and shoot it>
Look for copper "lines" or marks in the brakes bore.
Who did the brake install?
If you want us to help we need ALL the info............
Respectfully,
LG
 
#24
Re: 16 inch 308 keyhole problem?

+1 on the damn brake!!! Shoot it without the @#$%^&* brake installed and see if your problems disappear.

You said "[you] shortened [your barrel] to 16 inches..." Now, if you mean that you personally cut down the barrel and rethreaded it, you could have screwed it up! I am not calling your abilities into question, but threading a muzzle is an exercise in precision gunsmithing. If you thread the barrel and the threads are even a small amount off of being concentric to the bore, bullets can and will impact your muzzle device (even if extremely slight contact) and create instability (or WORSE). Again...this holds true for "SOME" gunsmiths who have no business doing certain types of work.

Once again...thread the brake off the muzzle and shoot the rifle without the brake installed to see if your problems go away.
 
#25
Re: 16 inch 308 keyhole problem?

I removed my brake and looked at the crown and the brake, there are no signs whatsoever of any signs of bullet strikes. The crown looks good also. I always get my barrel centered before I thread for a brake and have not had issues with any of the rest of the barrels I have done, I have brakes on almost all of my stuff. I am going to a match this weekend and will take this along with me (not using it) using my trg 42. I will try it with and without the brake and I am bringing some lighter bullets with factory loads.
 
#27
Re: 16 inch 308 keyhole problem?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sniperhandle</div><div class="ubbcode-body">I removed my brake and looked at the crown and the brake, there are no signs whatsoever of any signs of bullet strikes. The crown looks good also. I always get my barrel centered before I thread for a brake and have not had issues with any of the rest of the barrels I have done, I have brakes on almost all of my stuff. I am going to a match this weekend and will take this along with me (not using it) using my trg 42. I will try it with and without the brake and I am bringing some lighter bullets with factory loads. </div></div>

Tell us how you indicate for the MB and what readings are you taking and from where and using what to do so.
Respectfully,
LG
 
#28
Re: 16 inch 308 keyhole problem?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sniperhandle</div><div class="ubbcode-body">The main question I was trying to get an answer to, is: Does a shorter barrel need a faster twist rate or is it irrelevant? </div></div>

NO...GTG as is
 
#29
Re: 16 inch 308 keyhole problem?

Hello, lg, why dont you join us out at desert marksman this weekend for the thousand yard match, I would enjoy meeting you and it is only a short drive for you? we are leaving today, the match is on sunday
 
#31
Re: 16 inch 308 keyhole problem?

Ive been doing some searching on this issue, and this thread seems the most recent on the topic.

I was having a keyholing problem on a recently cerakoted .308 with a surebreak compensator. The problem went away after maybe 20 rounds. We ran patched down the bore every few rounds and that didnt fix it. The problem simply went away later in the afternnon, with zero keyholes thereafter.

It wasnt until cleaning everything up later at home that we figured out what happened. The cerakote coated the inside diameter of the last ring of the compensator. You could see that all of the cerakote inside the diameter had been forcefullly scraped off by the rounds just barely contacting the cerakote inside the outer diameter of the hole. Cerakote adds a few thou to the finish, so this, we believe in our case was what caused the issue. So, moral of the story, dont get any cerakote inside the bore or compensator.

hope this helps anyone with this issue in the future
 
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