confused grain vs. bbl twist & length question

[PA Rifleman]

I was in the local gun shop and overheard a interesting conversation pertaining to projectile weight and barrel twist and length. What I overheard was that the AR15 can be picky and certain bullet weights work best / better with certain barrel twist rates and barrel lengths.

I will soon be the new owner of a 16 inch 1:9 heavy barrel carbine so what projectile weight is recommended for best results in my AR?

This is not my 1st AR but it will be my first AR that I take seriously. I had 3 prior to this but all of them had issues in which I was pushed away and swore not to return but I'm back and plan on getting serious with this one.

[shootist~]

IMO, it;s better to say that quality bullets, including lighter ones such as 55 gr, work better than cheap ones regardless of twist. Some 1x9s may stabilize the cheaper 55s better, however.

A 1x9 twist "should" be fine with anything up to and including 69 grain bullets. Anything heavier, it's best to try a small sample and see first.

[5pins]

What brand of AR did you get this time? Most of the AR with a 1/9 heavy barrel are not the best quality. I’m guessing a Rock River or DPMS or something like that. If you can hold off and take some time here and look around then I think you could make a more informed choice.

Keep the bullets under 69grs and you will be fine.

[Clint]

In general, the longer the bullet, the more RPMs (faster twist) it needs to be stable.

Very short barrels give Low velocity and less RPM for any given twist.

[ICANHITHIMMAN]

What brand of AR did you get this time? Most of the AR with a 1/9 heavy barrel are not the best quality. I’m guessing a Rock River or DPMS or something like that. If you can hold off and take some time here and look around then I think you could make a more informed choice.

Keep the bullets under 69grs and you will be fine.

bingo first thought I had

[PA Rifleman]

I went with a M&A parts complete upper on top of an essential arms lower with RRA internals and a Bushy stock. I know its not exactly a top notch build but at this point I just wanted a cheap base model that I can use to learn what I want from it and rebuild to suit my purposes and comforts. I have a feeling its going to turn into an entirely different AR HAHA.

[Iraqgunz]

It's too bad that you wasted money on substandard stuff from M&A. In any case you should be fine with rounds from 55gr. up to 69 gr. If you want to shoot heavier rounds then you should purchase small amounts to test first.

I have to ask. What did you pay for that carbine complete?


I went with a M&A parts complete upper on top of an essential arms lower with RRA internals and a Bushy stock. I know its not exactly a top notch build but at this point I just wanted a cheap base model that I can use to learn what I want from it and rebuild to suit my purposes and comforts. I have a feeling its going to turn into an entirely different AR HAHA.

[polymorpheous]

I always found it curious that M&A parts is a site sponsor.

I built my very 1st AR from parts I bought from them at a local gun show.
It ran fine for a while, then I got a major stuck case.
I was waiting for something like this to happen, and I used it as an excuse to get rid of it.
I sold the whole damn thing and started over with a Daniel Defense upper.

[wahoo95]

I went with a M&A parts complete upper on top of an essential arms lower with RRA internals and a Bushy stock. I know its not exactly a top notch build but at this point I just wanted a cheap base model that I can use to learn what I want from it and rebuild to suit my purposes and comforts. I have a feeling its going to turn into an entirely different AR HAHA.

I assume your 3 previous AR's which didn't go so well were also made from suspect parts?

Sent from my DROIDX using Tapatalk 2

[C2_Drew]

Back on topic...

The formula for determining optimal twist rate is:

(CD^2)/L * sqrt(SG/10.9)

C is 150 or 180 if velocity is > 2800 fps
D is diameter of the bullet
L is length of the bullet
SG is the specific gravity of the bullet which should be 10.9 for lead core bullets. If that's the case, it cancels out the right side of the formula since the sqrt of 1 is 1.

So for M193 55 gr ball
(180 * 0.224^2 / 0.760 ) * 1 = 11.88

That's assuming the SG of the M193 is 10.9. Since the M855 has a steel core it will have a lower SG, and with further increase the twist rate needed.

M855 is 62 gr with a length of 0.907
M856 tracer is 63.7 gr with a length of 1.15
A 77 gr Sierra Match King is 0.994

Of those three the M856 requires the fastest twist even though it's more than 10 gr lighter than the SMK.

[shootist~]

C is 150 or 180 if velocity is > 2800 fps

Is the 2,800 fps value at the muzzle or at some distance downrange?
For v > 2800 I get 7.6 for a 77gr SMK, but < 2800 is get ~9.

Just trying to understand the math. My 77gr load runs around 2,750 fps.

[Bimmer]

This is not my 1st AR but it will be my first AR that I take seriously. I had 3 prior to this but all of them had issues in which I was pushed away and swore not to return but I'm back and plan on getting serious with this one.


I know its not exactly a top notch build but at this point I just wanted a cheap base model that I can use to learn what I want from it and rebuild to suit my purposes and comforts...

So, this is you finally getting serious... but you just want a cheap base model???

And the answer to your question is to get a rifle with a 1:7 twist, so that you can shoot virtually everything.

[PA Rifleman]

It's too bad that you wasted money on substandard stuff from M&A. In any case you should be fine with rounds from 55gr. up to 69 gr. If you want to shoot heavier rounds then you should purchase small amounts to test first.

I have to ask. What did you pay for that carbine complete?

$471.25 shipped for the complete upper assembly and $175 for the complete lower.

[PA Rifleman]

I assume your 3 previous AR's which didn't go so well were also made from suspect parts?

Sent from my DROIDX using Tapatalk 2 I had a bushmaster XM15 that suffered a broken carrier on the first day of a 2 day carbine course. a del-ton that didn't like to stay staked and a DSA that didn't like any ammo at all constant FTFs and FTEs and none of them liked steel cased ammo so I gave up and went with the AK but I decided to get back in on it.

[PA Rifleman]

So, this is you finally getting serious... but you just want a cheap base model???

And the answer to your question is to get a rifle with a 1:7 twist, so that you can shoot virtually everything.
Ultimately I'll end up with just the receiver from this one but I wanted something I can shoot and build up at the same time.