AR group problem

[itchyTF]

For all of you great AR diagnosticians!!

I’ve got an AR in 20 Practical and when developing a load I found that H4198 caused it to short stroke (settled on N133). John Hollinger (who did the original barrel work) said it might need to be re-ported which he did. (This info is not necessary, just background.)

Had it out last weekend for testing and noticed that the first shot of every 5-shot group was high. I thought the very first shot might have been the result of a cold bore. I loaded up the next two 5’s and it did the same thing. The barrel was not really that warm at this point but I waited about 10 min before doing the next two sets of 5 and they did the same thing. Although, the “high” one with 4198 was not that high. (No more short stroking.)

(I realize the groups are nothing to write home about but my excuse is I had wind and mirage!)

The only thing (I can think of) that is common with all of them is that the first shot of every group was loaded by releasing the bolt stop. One experiment I need to try is to fire a group where every shot is loaded that way. If they all end up high, that will still leave me with – why? Any other thoughts?

Range was 100 yds., target grid is ¼”.

AR –
EG&G scope rail
Sightron SIII 6x24
Free float tube
24”, 1-11 barrel
Geissele DMR trigger

 

[KMart]

Try increasing the neck tension to keep the bullet from moving forward when the bolt slams home. It sounds like the force generated on just releasing the bolt on the first shot is not as great as when the rifle cycles on its own.
I had similar problems on a 223 AR and the problems went away when I went to about .004 neck tension.

 

[Beau]

Size your cases to .003" shorter headspace than fired cases.

 

[itchyTF]

Beau -
I use a .010 Redding shellholder when I full length resize which gives be about .003 shoulder pushback.

KMart-
I don't neck turn these cases (nor do I plan to) so I use a carbide expander button which yields me about .0025 of neck tension.
I have used an inertia type bullet puller with these rounds before and I had to beat the s__t out of it to get the bullet to come out.

With the low mass of the bullet I think .002 of neck tension is enough. There is no way the bolt slamming forward is harder than what I had to do with the puller.

 

[KMart]

If you turn the necks and use the same sizing die, you will decrease the neck tension.

Let us know what you find out.

 

[Recko555]

ItchyTF,
I have almost the same setup as you.
WOA 20 Practical
24" Pacnor 1:10 twist barrel
Sightron 6-24x50 Scope
LaRue Mount

I experimented with H4198 and didn't like the results, so I settled on N133. I started with with a .226 bushing and wasn't happy with groups, so I went down to a .225 bushing (and expander ball) and N133. I am very satisfied with this combo (I had to go down in powder charge slightly with the tighter neck tension). I had a buddy get a carbon copy upper of the above set up. I reload and sight in for him and he had none of the symptoms you describe. I use 10X powder for his ammo and it works great too but is a little dirtier (doesn't want to spend the extra for N133).
I also seat my ammo to 2.250-2.255.

I would:
Single load all ammo and shoot for groups.
Have someone else shoot for groups single loading and mag loading.
Try longer OAL of bullets.

Also, what bushing size are you using?

 

[itchyTF]

My loaded necks run about .226 and the expander is .202? Don't remember the 4th place. I use a .221 bushing which would bring down the ID to around .199-.200 (my guess since I don't have a ball mic or pin gauges) and expand back up to .202. Don't know how much springback is involved. I suppose I could use a .223 bushing and work the brass a little less.

How much difference in springback is involved when using different bushings and an expander? If the neck is sized down to an ID of .198 versus .201 with a .202 expander, does the ID come out the same?

I like to keep my OAL to 2.250 or less for mag clearance. The tips are barely missing at 2.250.

KMart -
I don't think I'd get enough benefit turning necks for this cartridge. I turn for a 6x47L and .243AI. There is also a lazyness factor. I can put up with some level of tedium if I suspect a benefit. But I have my doubts on this one.

 

[Recko555]

If you have a .225 or .226 bushings, I would try that. I haven't heard of people using that small of bushings before. I have heard of larger bushings like .230.

 

[Klong]

Use a different bullet.

 

[itchyTF]

Recko-
If my loaded necks are running .226 and I use a .226 bushing my neck tension would be next to nothing.

 

[people]

It is probably neck tension. You can do the single load test or load a mag of 10 rds and shoot a group of 5 then just sit there for a minute or 10. Just sit there then fire the remaining 5 shots. If those last 5 are the same as the last 4 it is tension. You can also load a mag of 10 then chamber the rifle drop the mag and shoot. Do this 10 times and if it never settels down. You know it is the neck tension.

 

[Recko555]

Then use the .225" bushing. Different brands of brass have different thicknesses, that is why different bushing are needed to get correct neck tension. Below are some sources on the 20 Practical I am sure you have read, but they give good information on bushing sizes. Warren in the below article uses .228" bushing.
http://www.accurateshooter.com/guns-of-week/gunweek061/
http://www.6mmar.com/20_Practical.html