Beginner Load workup on AR-15, 5" differential between best and worst group???

I worked some loads up today for an AR-15. My group sizes ranged from 7/8" w/ 22gr of powder to nearly 5.75" with 24.3gr powder at 100yds. Can group sizes change this much due to barrel harmonics or is this more or less shooter error, etc.? If I were to consider the 7/8" group as being zero harmonic error and the 5.75" group to be harmonics, that would make the barrel vibrating about +/- 0.001 inches due to harmonics if these were the outer most spread. Does this make any sense in terms of this much of a difference? By the way, this rifle is a Delton 20" AR w/ YHM FF handrails and a standard CS, 1x9 twist barrel. I shot off of a bench using a Tango Down bipod and using my left hand as a rear rest. Any opinions?

I loaded with 68gr Hornady BTHP, Ramshot TAC powder, Wolf Small Rifle Primers and the outside temp. was 50F. Here is the data (I also included a 10round group of 55gr HPBT below for comparison). I can attach images if needed. Thanks.

68gr. Hornady BTHP, Ramshot TAC, Wolf Small Rifle Primers (outside temp = 50F)
Charge––––––––––––Group––––––––––Sizecomments
gr Powder––––––––––inches
22––––––––––––––––––0.875––––––––––-5 shot group, no pressure signs
22.5––––––––––––––––3.75––––––––––––5 shot group, no pressure signs
23––––––––––––––––––3.625––––––––––-5 shot group, no pressure signs
23.5––––––––––––––––1.25––––––––––––5 shot group, no pressure signs
24––––––––––––––––––3.5––––––––––––––5 shot group, no pressure signs
24.3––––––––––––––––5.75––––––––––––4 shot group, no pressure signs
24.6––––––––––––––––2.375––––––––––-5 shot group, no pressure signs
24.8––––––––––––––––2.75––––––––––––-3 shot group, no pressure signs

55gr. Hornady FMJ-BT, Ramshot TAC, Wolf Small Rifle Primers (outside temp = 50F)
Charge––––––––––––Group Size––––-comments
gr Powder––––––––––inches
23.5––––––––––––––––2.5––––––––––––––-10 shot group, no pressure signs. Powder can vary +/- 0.5gr.

Check your scope. It might have shot loose.

After you check your scope I would check and make sure the bipod is not causing problems between the barrel and handguard. Then you could re-shoot those groups and see if the same thing happens, if it does then you are on to something there, if not then look elsewhere. In any case you are working your loads up correctly.

Bob

Thanks, I'll check that and re-shoot this weekend!

7/8" for an AR sounds good to me.

What sort of setup do you have ?

What barrel ??

Do you have a freefloat handguard or a regular handguard.

Flat top or carry handle with a scope mount.

What scope ?

None of my Ar15's were particularly accurate even with a match barrel.
You can bed the barrel and that helps.

All that said, it might be more the timing that is thowing
off your groups more than anything, if everything else is perfect.
Tubbs has some sort of gizmo that holds the lockup slightly longer and is supposed to help accuracy.
Never tried it myself.

In the future try some 52 gr amaxes over about 26.5gr - 27gr of varget.
Caution, very hot load so work up.

My Bushmaster 1-9 does not like heavy bullets and it's a 24 inch. I settled on 27.2 grs. IMR 4320 with a 55 gr Horn. SX. Had a bunch of these loaded for a bolt gun. Never clocked them threw the AR but they went 3240 in a 24" Varmint Synthetic on a 50 degree day. They will do 1/2 inch 5, bagged up good, if I do my part. They are very violent even at long range, plus in my gun they won't black out a 14 power scope. Does your gun have a good trigger? That was always my complaint with ARs until I got the Bushmaster. It has a very nice 2 stage trigger. I shoot it bone stock. I hope this helps.

If not a loose scope, my vote would be the trigger. If you're using the "average" LPK trigger out there, it makes tiny groups a real challenge. I can recommend the Jewell AR triggers wholeheartedly. Cmmg trigger wasn't too bad, but not as nice as the Jewells. I bet those same rounds shrink up with a nice trigger.

Quote:

---

Originally Posted by tammons

7/8" for an AR sounds good to me.
Sounds really good to me, I was thinking it would be somewhere around 2moa, which the 10rd group of 55gr FMJ-BT at 2.5" sounds about right, especially off a bipod and using my hand for a rear rest
What sort of setup do you have ?
It's a mega machine lower on a delton upper, probably a DPMS upper, I also have a RRA 2 stage match trigger installed
What barrel ??
standard 20" chrome moly barrel, 1x9 twist, probably a Wilson or ER Shaw
Do you have a freefloat handguard or a regular handguard.
YHM Freefloat handguard
Flat top or carry handle with a scope mount.
Flat top w/ Larue Mount
What scope ?
Wotac Scope

---

I'm just wondering if there can be nearly 5" of delta attributed to a different powder load, e.g. barrel vibration, I wouldn't think so, but I don't know.