Gunsmithing Some short barrel questions for the pros

[sawman556]

Hey all. Here is my idea. If I'm crazy please let me know

I have a Stiller TAC 30 with a 20" Krieger varmint/Sendero profile in a KRG W3 folder chassis.

I was thinking of re-barreling with a Krieger #10 cut to 16.5". Match chamber in .308 and threaded 5/8x24. I would also what to flute it to shave some weight and look cool.

My questions are would this setup work well for a 600 yard gun? Would I want 1:10 over 1:11 or 1:12? What kind of velocity could I expect from 168 FGGM pills? Or would I need to go with lighter projectiles? Any idea what the barrel OD would be a 16.5"? Would the recoil / muzzle rise be more with a short barrel? Would there be enough gas pressure to make a muzzle brake work effectively? Would it still be capable of sub .5 MOA like my 20"?

I'm looking to build short bolt action that could be used as a NJ carbine. I am not interested in building a neutered AR. Is the a viable idea? I will build a long action in the spring in a better caliber for 1000 yds and beyond.

Thanks. Any help would be greatly appreciated.

 

[Jedi]

Re: Some short barrel questions for the pros

Certainly a viable idea, several here run sub 20" barreled 308.

Short fat barrels are extremely stiff n less subject to harmonic issues, so should shoot a wider range of loads better in theory.

Dont see any issue with brake, regardless of length gas has to exit bore via muzzle

I would run hot 155, have chamber cut per your dummy round
load em long n stuff with vargay ... 47.8 is a known load
Tweak as needed n enjoy

 

[Iggy]

Re: Some short barrel questions for the pros

I would maybe suggest an 18" barrel? Are you running a suppressor?

 

[KrazyL]

Re: Some short barrel questions for the pros

I run a 16.5" Savage some times. I can maintain about .5 MOA with FGMM 168's all day. I run suppressed, and have taken it out to 800+ yards with no problem. Only time I ever have a problem is when there is a loose nut behind the trigger...

 

[LRI]

Re: Some short barrel questions for the pros

The shorty 308's we've built don't seem to be very fussy at all. 308's in general are about as forgiving as they come so the deck is sorta stacked in your favor.

FWIW years ago Middelton Tompkins did some testing with the Army and they learned that a 168 Sierra in a 1:12 barrel was a super accurate combination out to 300 meters.

This was back in the day when 300M 3P was still an Olympic event, but the data is still valid today.

I say go for it. Shoot 175's or 180's and don't look back.

Good luck.

C.

 

[xs hedspace]

Re: Some short barrel questions for the pros

47.8 gr Varget?? Hope you moly the 155 gr BTs. I see ejector marks on bare 155 gr load cases with 46.5 gr Varget. But I have a .3075 Krieger barrel. Work up slowly.

 

[LRI]

Re: Some short barrel questions for the pros

47G of Varget is a pretty hot load for 155's. Some Palma guys run them that high, but if your trying to do it with a heavier bullet I think you better bring a mallet with you to the range for your bolt.

C.

 

[anthonylapoint]

Re: Some short barrel questions for the pros

If you search enough, you'll find lots of articles where guys are running 18" out to and past 1000 yards easily and loading a lot more mild then 47 grains of varget. Go on beyond 1000 here on the hide and youll see that. 16.5" can shoot .5 without a doubt and a muzzle break would work well as youre getting the same muzzle blast but you dont have 6-8"s of barrel/weight to keep the jump down. If youre going suppressed then all that doesnt matter as the suppresor will help all of that. I believe the krieger #6(or#7) fluted weighs the same as a #4. Go 1-10 twist and shoot heavy to keep your velocity slow if youre trying to go subsonic for the suppressor but yet keep your energy up to punch through targets. (think 300 blackout) Just a thoought but theres a thousand ways to skin a cat

 

[Pusher591]

Re: Some short barrel questions for the pros

I ran a 17.5" barrel for the last year and made several hits at 1050 yards. I consistently shot at 730 yards on a friends range and groups usually were around MOA or less. You should be good at 600 with only one less inch

I do suggest shooting the heavies, and particularly the Hornady 178 BTHP's
They were noticeably better then the 175 SMK's because of the BC advantage.... at least I could tell a difference.

McCourt Ammunition loaded mine and it printed 1/4-1/2 moa at 100 yards and always held MOA out to 730 yards.

 

[sawman556]

Re: Some short barrel questions for the pros

Thanks guys. I was just thinking about having a shorter rifle that I could use as a carbine. There is pretty much no restrictions on bolt actions in NJ other then the 15 round mag limit and no 50BMG's. I can only get out to 600 on my range though we will be working on a 1k range in the near future. I want to build a long action next spring that would be a better fit for 1k anyway.

So 1:12 with 175's?

I there a considerable difference in the amount of muzzle rise form a 16.5 compared to the 20 I am running now?

I was also thinking of piggybacking a RMR of my NF NXS for close range quick shots. Any thoughts on this?

 

[2156SMK]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawman556</div><div class="ubbcode-body">So 1:12 with 175's?</div></div>

You might run into problems with a 16.5" 1:12 twist barrel.
Atmospheric conditions WILL be a factor here as you are right on the ragged edge of where it will work and won't work.

Only way to know is to try it...

 

[LRI]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 2156SMK</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawman556</div><div class="ubbcode-body">So 1:12 with 175's?</div></div>

You might run into problems with a 16.5" 1:12 twist barrel.
Atmospheric conditions WILL be a factor here as you are right on the ragged edge of where it will work and won't work.

Only way to know is to try it... </div></div>


Being serious, how come?

As I envision this, a bullet goes from zero to prescribed rate of twist regardless of velocity. RPM certainly does vary with velocity, but rate of twist doesn't (right?)

I mean the bullet rotates one time per distance traveled regardless of how fast its going right? Bump velocity and now its just doing it more times per minute than at a slower pace.

Assuming I'm correct the shorter length just means the potential for less velocity since there's less "time/distance" (this is starting to sound like a Back to the Future movie) to accelerate the bullet which would then of course mean its not spinning fast enough to stabilize. (?)

Right?

Time to go chain smoke. . .

 

[LRI]

Re: Some short barrel questions for the pros

So on that leap of faith if we were to have some hypothetical means of accelerating a bullet to ludicrous speeds (lets just pretend and say 5500fps) we could in theory slow the ROT way down and still achieve the target RPM that it takes to properly stabilize the bullet.

Example:

A 175 coming out the door at 2850 in a 1:12 rotates at 171,000rpm if my math is right.

So, if we need 171,000 rpm to properly stabilize a 175 that would mean a twist rate of 1:23.1 at 5500 fps.

Right?

 

[sawman556]

Re: Some short barrel questions for the pros

You guys are burning it on both ends!!!

 

[Gene Poole]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: C. Dixon</div><div class="ubbcode-body">So on that leap of faith if we were to have some hypothetical means of accelerating a bullet to ludicrous speeds (lets just pretend and say 5500fps) we could in theory slow the ROT way down and still achieve the target RPM that it takes to properly stabilize the bullet.

Example:

A 175 coming out the door at 2850 in a 1:12 rotates at 171,000rpm if my math is right.

So, if we need 171,000 rpm to properly stabilize a 175 that would mean a twist rate of 1:23.1 at 5500 fps.

Right?

</div></div>

I don't think so. JBM (with numbers it accepts as valid) says:

4000 1:20 = 0.399
2000 1:10 = 1.266

 

[2156SMK]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: C. Dixon</div><div class="ubbcode-body">Being serious, how come?</div></div>

The velocity plays into stabilization because with increased velocity (barrel length gives velocity), you get more rpm's.

It's not a specific twist rate that stabilizes the bullet.
It's an RPM.

You can get away with slower twist rates on longer barrels because of the velocity.
Think about all those palma guns you build with 1:13 and 1:14 or even 1:15 twist for the high elevation guys...

The sames rules apply to shorter barrels.
So while we can get away with 1:12 twist on our 24" 308 barrels and shoot 175's and 178's, I'd be willing to wager a beer than the 16.5" barrel is gonna have issues with 175's and 178's, especially in low density altitude conditions.

 

[wwood]

Re: Some short barrel questions for the pros

Keep in mind that the rotation of the bullet is to create gyroscopic stability which counters the aerodynamic forces acting to destabilize the bullet. The gyroscopic stability is a function of the physical properties of the bullet ( length, mass,etc.) and its spin rate which is determined by the barrel twist x speed. The aerodynamic forces will also be a function of the bullets velocity, and they will tend to be non-linear. You can see that in the JBM results Gene Poole posted. As you increase the bullet velocity, the aerodynamic forces increase non-linearly and the spin required to counter them will increase non-linearly, but the actual spin out of the barrel will only increase linearly. Assuming you start with a twist rate/velocity/bullet combination that is over stabilized, you may still be able to increase the muzzle velocity to a point where the bullet no longer stabilizes.

 

[azimutha]

Re: Some short barrel questions for the pros

Using JBM's stability calc, I ran the Berger 175 OTM (1.268") and would feel comfortable using an 11.25 twist on a short barrel. I set MV @ 2300 fps, the temp down to 0 F and left the pressure high (29.92). Stability was still 1.513, which for that bullet design would probably be more than adequate for 600-800 yards. In my typical conditions, using a 24" bbl. and 2650 fps, the stability factor for that bullet is over 2.

 

[VKC]

Re: Some short barrel questions for the pros

I would go with a 1:10 twist with that short of a barrel, for better stabilization.

168 or 175 should be fine.

There would not be much more recoil or muzzle rise with a short barrel, just get a good solid position. Nevertheless, I would use a muzzle brake myself, more so to spot my own shots...something lower profile like a R&D or RAD brake.

A shorter barrel with the same OD will actually be more accuracy than a longer barrel, as it is stiffer.

600 yards with a 16.5" barrel should not no problem if your handload.

 

[Freeze]

Re: Some short barrel questions for the pros

I run a 16.5 rock 1-10 twist. I am shooting 175 Berger vld's at about 2520-2550. It shoots very very well. One group...and I do say one cause its not an all day long scenario, it only happened once, but I had a .24 inch group, yes inch group not Moa, at 200 meters. The accuracy in the gun is there, now if I could only do that all the time and not once In a blue moon. It's a fun gun, very handy and discrete in an aics 2.0 stock.

And just more group clarification so I don't get jumped on, was only a 3 round group cause 1, I was testing an old load to see how it would do in the new barrel, and 2, I was a damn pussy and didn't want to screw it up.

 

[19Scout77]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawman556</div><div class="ubbcode-body"> There is pretty much no restrictions on bolt actions in NJ other then the 15 round mag limit and no 50BMG's. </div></div>

50bmg of both the bolt action and semiauto variety a perfectly legal in nj.

 

[sawman556]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 19Scout77</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sawman556</div><div class="ubbcode-body"> There is pretty much no restrictions on bolt actions in NJ other then the 15 round mag limit and no 50BMG's. </div></div>

50bmg of both the bolt action and semiauto variety a perfectly legal in nj. </div></div>

Sorry I'm new to this state. Seems I've been lied to. Thanks.

 

[2156SMK]

Re: Some short barrel questions for the pros

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: vkc</div><div class="ubbcode-body">I would go with a 1:10 twist with that short of a barrel, for better stabilization.</div></div>

This would be my advice as well.

 

[sawman556]

Re: Some short barrel questions for the pros

Now I can't find a damn barrel

 

[300sniper]

Re: Some short barrel questions for the pros

my 11.25 twist, 18.5" barrel has stabilized everything i have run though it. granted i've only tried most of the 155 grain flavors, 168 smks and 175 smks. even the 175 smks were coming in point first at 1500 yards so i'm thinking it is just fine.

2" shorter barrel? i am not sure if it would require a faster twist or not. i personally don't think i'd go shorter than 18" though. i find my 18.5" barrel 308 is "snappier" than say my 27"+ 284, even though they are built the exact same way. in my opinion, the shorter rifle is less forgiving to poor positioning (you can't always get into a perfect position) than the longer barrel.

with my 18.5" .299 bore bartlein, i was getting 2600+ fps with fgmm 168 ammo. it shoots it really nice too.