Do I need a heavier round?

[Sierra1926]

Hey, I am new to the site and look forward to learning more. I got a question and wasn't sure where it belonged. If this is the wrong thread, I can move it.

The rifle I built is a RRA upper/lower with a White Oak 24in 1:8 varmint barrel that is floated, with a 3.5-10 scope. I was shooting Fed 55gr TRU ballistic tips at 100 yds and it would only hold about a 3-4 inch group. Is the twist too fast for the light round or should I try a heavier round?
Again I am still learning and I thank ya'll for any help.:)
Sierra

[Jake'sDad]

That same load shoots under an inch in my BCM 1-8 twist SS upper I got from Grant, even with me shooting, so either your gun doesn't like it, or there's some other problem.

BTW, that load is a great varmint load, but a lousy defense load, just FYI.

[Belmont31R]

Try some open tip match (OTM) bullets like Sierra Match King, Nosler, Hornady, ect.



55 grain rounds can be accurate but Ive noticed they are more susceptible to wind than heavier bullets in the high 60 to mid 70 grain weight.

[Microalign]

1:7 twist rates will stabilize 55gr bullets just fine. So, a 1:8 twist rate is no problem.

If you want to calculate it, the Greenhill calculation for determining the slowest twist rate for bullet stabilization is:

Slowest Twist Rate = 150/(bullet length/bullet diameter) x bullet diameter

Most 55gr bullets are around .647" in length, and .223 Reminton is actually .224".

150/(.647/.224) x .224 = 11.62...........or 1:12 twist


With your 1:8 twist rate, you should be able to shoot bullets as heavy as 77gr with no accuracy problems. My guess is that your setup has some spec issues. I've never been a big fan of frankensteining parts from different manufacturers, putting them together to trying and make a precision rifle. You might have a head space issue between the bolt carrier group and the barrel, the barrel/chamber might be out of spec, gas leaks, .....etc. I would try some different types of ammo first and see if that corrects the problem, after that, you should consider locating a reputable AR-15 armorer in your area, or better yet, send it to G&R Tactical for a diagnosis.

[Sierra1926]

Thanks for the info guys !
Jakesdad and Micro, ya'll probably got it correct.....probably more an issue with me than the rifle. I'll give it another go with the 55s and carry some 77s with me and try those also. If I can't get it to tighten up, I just might send it to G & R.
Thanks again for the help. I'll report back with result from the range.

[mtrmn]

Probably just an issue with that certain load not jiving with your barrel flex/vibration node. Every gun is a little different in what it likes/dislikes. Try it again and if it doesn't work, just try another load. Just like in reloading, a different powder or powder charge will make a world of difference.

If I understand this correctly, the bullet must exit the barrel at the same point of barrel flex every shot in order to be accurate. Otherwise your groups will string vertically.

On the other hand, if the shooter is at fault, the groups will usually string horizontally. I may be way off on this, but I've had good results with this approach for over 20 years of reloading/shooting.

All this assumes everything is tight on the weapon.