Let me start by saying this was a very small sample size and the statistical significance of these values can be argued six ways to Sunday. I am shooting this in the prone position with a sling and iron sights. No other support or optics.

I have always loaded AR competition loads (23.2 gr. of 8208XBR, LC brass, Wolf Primer) for rapid fire to about 2.255” oal. This load holds the 2 MOA 10 ring if not the 1 MOA X ring in a service rifle target easily if I do my job. A while back a fellow competitor said he ran a test and showed that if he seated shorter than published OAL to meet magazine length he had tighter rounder groups in the rapid fire stages of the match. I never gave it much thought as the conventional wisdom is that the 77 grain HPBT match were not sensitive to jump at all. Just load at mag length and shoot it. I know VLD secant ogive bullets are very picky for jump but these 77HPBT bullets are pretty robust and not picky at all Right? I thought I would run a quick and dirty test to see what OAL my new barrel “liked “ best.

I was shooting at 100 yards at an SR-1 target. I shot 10 shots with 2.250” OAL in the center of the target and had a 2.0” group as a control.
I then cranked on the iron sights and put 20 clicks left windage and +25 elevation on the gun. I fired five shots of the load seated to 2.260” OAL. The aiming point and sight picture was the same as my control group but the bullets printed in the upper right hand corner of the target out in the white scoring rings.

Next I came down 25 clicks and shot 2.250" group. This printed at the X ring level but in the white at 9:00. The 5 shot group at 2.250” was also 2.0”. So… the group size of a five shot group is representative of what a ten shot group is.

I then came down 18 (to dead bottom) and fired 2.240" OAL group which printed in the lower left corner of the scoring rings in the white.

I then added 43 up (18+25) and 40 clicks right to get to the upper right corner and the 2.230” group was fired.

Down 25 to mid-line of target on 3:00 side for 2.220” OAL.

Down 18 to dead bottom for 2.210” group.

The results are shown in the table below for all five shots and the best four out of the five.


COAL 5 shots Best 4
2.260 1.2 1.1
2.250 2.0 1.2
2.240 1.2 1.0
2.230 1.2 0.5
2.220 2.2 1.5
2.210 2.2 2.2

The two real short ones are clearly not as good as the others. The 2.250” is not bad, but not as good as the other three. 2.260 2.240 and 2.230 are about the same in 5 shot measurements. The 2.230 groups roundness was better and half the size for best 4.

In the end… given this small sample… All the top 4 are really about the same. BUT – I will be loading 2.235” just in case.

hmmmmm, keep us informed.

Dawgdad wrote:

... 20 clicks left ... +25 elevation .... down 25 clicks ... down 18 ... added 43 up (18+25) ... Down 25 ... Down 18 to dead bottom

Wow, glad I completed higher level math! I will to go through this again.

Ok, caught my breath. Your rifle is doing pretty good. I've seen some report 2" variation in group size just due to COAL differences. COAL adjustment is the last step in load development that I perform to fine tune a load. Its particularly beneficial with Barnes TSX/TTSX bullets. They are even finding that Berger can benifit from benig further from the lands.

I just started a project on my 6.8mm AR to load up to 2.425" COAL. I modified my magazine and recievers to accomodate this. Most 6.8 bullets are way too far from the lands. I've already found much better accuracy. I've done this for hunting.

68forums.com/forums/sh...zine-COALs

Why the long throat on tne AR? You would think that with so many manufacturers someone would make a barrel to remedy this..

The military specifies a long throat in order to get the higher velocity and handle the higher pressure loads. That is the main difference between a 5.56 chamber and the .223 chamber.

My rifles have a 223 Wylde chamber which has a long throat too like the 5.56 to accommodate 80 grain bullets loaded longer than magazine length which are single loaded longer distance shooting. (600yds.)

dhc4ever wrote:

Why the long throat on tne AR? You would think that with so many manufacturers someone would make a barrel to remedy this..

I was addressing the issues with the 6.8mm SPC. Remington designed this cartridge but got the throat, primer size, and barrel twist wrong. The SPC II solves many of those issues and AR15Performance makes a 6.8 barrel with a shorter thoat because most of the .277 bullets used in the 6.8 have a shorter ogive than those used in the .270 Win.

Well keep the info comming, you guys get to play with all the good toys, we all live vicariously through you lot since 1996 when most semi autos were banned. If you have a "genuine" reason and can jump through hoops like a trained seal, it can be done, but not easily for the general shooter.