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.30 cal. rifling question
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  blackbikesteve

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Posted: April 10 2017 at 5:12am | IP Logged Quote blackbikesteve

From reading in various sources, I have found that most .30 caliber firearms(7.62/.308 rifles), seem to have barrel twists of  around one turn in 10 inches, give or take an in. or so. This also appears to be true of the 7.62x25 Tokarev pistol and smg rounds. One peculiarity that I noticed was that the .30 cal. M1 carbine has a twist rate of one turn in 20 inches. I noticed this while doing research for a possible home built rifle in this caliber, and found that  practically no barrel makers offer this twist rate in a .308/7.62 barrel. I am just curious as to why 1 turn in 20 in. would be desirable  when i've read of "plinker" rounds of around 100 grains being used effectivly In .308, 30-06, etc. with good accuracy. Can anyone tell me why it might not be a good idea to use a basic 1 in 10 twist barrel for the project I have in mind. Is it an accuracy issue, or a safety issue? Anybody know? I'd appreciate the input. Thanks!
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  backbencher

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Posted: April 10 2017 at 4:57pm | IP Logged Quote backbencher

I'll give examples from .22" LR & .223", as this will perhaps best illustrate the issues involved.

.22" LR is normally shot through a 1:16" twist bbl.  This works well, but suppressor manufacturers generally won't warranty their cans if Aguila .22" SSS is used.  Why?  B/c .22" SSS has a long, 60 grn bullet, and doesn't always stabilize in 1:16" twist bbls, which can result in baffle strikes.

.222" normally has a 1:14" twist bbl, and since .222" couldn't quite make the armor piercing requirements the Army was demanding, the M-16 was chambered in what became .223".  The M-16 too had a 1:14" twist bbl, until the infamous Arctic tests resulted in a change to 1:12" twist, b/c it was alleged that some bullets in the Arctic test keyholed in the cold air.

Off to Vietnam w/ the 1:12" twist the M-16 went.  Colt brought out the SP-01 for civilians, also in 1:12" twist.  This was perfectly fine for the 55 grn round of M193, and the USAF discovered, also decent for .22" LR, which saved them a bundle of money in training.

In 1982, the USMC finalized the M-16A2 with a 1:7" twist.  Why so fast?  B/c NATO was switching to the 62 grn SS109 bullet in M855 ammo - but that stabilizes just fine in 1:9" twist.  However, the M856 tracer round that tracks similarly to M855 is 63 grains - but much longer, necessitating the 1:7" twist.

So let's take our .22" LR adapter from our old SP-01 with 1:12" twist, and plunk it into our new Bushmaster w/ a mil-spec 1:7" twist bbl.  Then let's shoot the same cheap bulk .22" LR out of it we always have - and discover our groups have gone to hell.  What on earth?  I thought we couldn't "overstabilize" a bullet?

Well, yes, and no.  What's happened is we're using cheap bulk .22" LR ammo - and the manufacturer doesn't spend very much to make sure those .22" bullets are perfectly concentric.  Spin them out of a 1:16" .22" LR bbl, they do fine.  Spin them out of a 1:12" M-16A1 bbl or a SP-01 bbl, there's hardly any difference in the groups, and any difference you'll chalk up to the gap btwn the bbl and the chamber adapter.

Spin those same cheap .22" LR bullets really fast out of a 20" 1:7" twist bbl, and any imperfections in the cheap bullets cause them to wander all over the place.

.308" Winchester will run all the way up over 200 grns, so needs a 1:10" twist with normal loads & velocities.  The M1 carbine is firing only a very short 110 grn pistol bullet - so it can get away w/ a much slower twist.  1:10" twist won't hurt your project - but you might give up a tiny bit of accuracy over a slower twist.  It would, however, allow you to shoot much heavier bullets, possibly setting you up for a subsonic .30" Carbine load.
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  blackbikesteve

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Posted: April 10 2017 at 5:20pm | IP Logged Quote blackbikesteve

Thanks for the reply backbencher, it does help. My confusion rose from seeing twist rates for the 7.62x25 cartridge at 1twist in 9.45 inches. I guess i might have to spring for a custom 1 turn in 20 barrel, or possibly a one in 17 rifled barrel, as I have seen that twist on some posts elsewhere as being optimal for accuracy for the .30 carbine round, if I actually do this project. Guess I still have some research to do…Thanks again!
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  MatthewMachinist

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Posted: April 13 2017 at 3:02am | IP Logged Quote MatthewMachinist

Blackbikesteve,

I agree totally with what Backbencher said, but I disagree with the conclusion you have drawn (buying a custom twist rate).

Your question leads to a discussion on over stabilization, over stabilization is only ever a problem if it is the cause of a problem.

There are two problems that I have encountered
1 Poor accuracy due to lack of concentricity
2 Lead smoke (where the round disintegrates in mid air)

Basically these are only ever a problem if you massively over stabilize (usually at very high velocity also) or if you use cheap ammo.

If you are planning on using good projectiles, I doubt you will have a problem.

Traditionally under stabilization has been regarded as far more detrimental to accuracy that over stabilization, however I have talked to some top long range shooters lately and I am surprised that they seem to get very good accuracy with projectiles well heavier than the recommended weight for a given twist rate. This is because they are shelling out for the very good projectiles, not the cheapies.

Bottom line is if you are using good quality projectiles you will be very surprised what you can get away with.

An old adage regarding rifle accuracy is that it comes down to the three B's... Barrels, Bedding and Bullets. If you don't skimp with those you should have a tack driver.

If you are planning on using cheap FMJ or surplus ammo then by all means do some more research and take my opinion with a grain of salt.

Hope this helps,
Matt.
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  MatthewMachinist

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Posted: April 13 2017 at 3:13am | IP Logged Quote MatthewMachinist

I just realized after writing that last post that I already have a gun that fits exactly into this category, my 300 BLK AR-15 is running 110gr Hornady ammo, no problems at all with 1 in 10 twist.

http://www.hornady.com/store/300-Whisper-110gr-vmax/
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  blackbikesteve

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Posted: April 21 2017 at 3:05am | IP Logged Quote blackbikesteve

To all of you that have replied to my original posting/question, Thank you very much for your time and thoughtfulness in sharing your opinions and personal knowledge on this subject. I shall keep it all in mind if I do attempt the screwy project i'm considering.
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