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末末末末末末Group末末末末末Sizecomments
gr Powder末末末末末inches
22末末末末末末末末末0.875末末末末末-5 shot group, no pressure signs
22.5末末末末末末末末3.75末末末末末末5 shot group, no pressure signs
23末末末末末末末末末3.625末末末末末-5 shot group, no pressure signs
23.5末末末末末末末末1.25末末末末末末5 shot group, no pressure signs
24末末末末末末末末末3.5末末末末末末末5 shot group, no pressure signs
24.3末末末末末末末末5.75末末末末末末4 shot group, no pressure signs
24.6末末末末末末末末2.375末末末末末-5 shot group, no pressure signs
24.8末末末末末末末末2.75末末末末末末-3 shot group, no pressure signs

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