Hornady GMX 70gr. in .223 and general monolithic reloading

[Ape_Factory]

I purchased three boxes of Hornady's 70gr. monolithic bullets, the GMX series, only to discover that Hornady says they won't stabilize in a 1:8 twist barrel. Unfortunately this is exactly what I have, 16" Wylde chamber, 1:8 twist in an auto-loader AR-15.

I loaded five rounds, H335@23gr. even just to see if I'd get keyholes at 100 yards. Nada. Seemed stable and I'm not sure why Hornady says they won't stabilize as I've shot up to 77gr. out of this barrel and all were fine. My other test load that day was Sierra 69gr TMK's and they too produced nothing but round holes.

Anyway, at that charge which was just a guesstimate starting point, something safe, middle of the road, I had an ES of 39 and a "respectable" SD of 17.3. So not a bad starting point for a guess. I have read a few other posts here on monolithics and most agree they like to run hot which has also been my experience.

With that said, I've loaded two different GMX weights in .308 and I get very inconsistent results day to day, despite using the same tools and techniques. I'll get a super tight group at 100 yards and the next time out, it won't produce nearly the same results. I've gone through an entire box of the 150gr. GMX bullets trying to find consistency with no luck. Last time out, I did manage an SD of 14.1. but not a tight group. Maybe me? Dunno. Same with the 165gr. GMX, had an SD of 10 with a very light charge (39gr. of IMR4045) and velocities in the mid 2200 range out of a 16" barrel. Not very hot indeed. They did not group this time out. I don't use a sled, just a cheap bipod and a sand bag. I've run that same charge before and they grouped much tighter but with a slightly higher SD.

I have started shooting the GMX first when I go to test. Typically I'll take a few different bullet types that I've loaded up, from the monolithics to normal, jacketed, lead-filled rounds. They'll either be ladder charges if I'm trying to find a node or it'll be a "proof" test with ten rounds loaded. I don't clean the barrel/run a bore snake in between bullets but again, I'm running the GMX first in most cases and I'm also going for practical accuracy, better than what I'd get with off the shelf loads.

So what is it with monolithics that makes them more difficult than normal bullets? Any reloading tips I need to pay particular attention to? I have messed with seating depth a bit but I feel like, other than overall case length, I have things under control. In fact, this time out I had for the first time single digit SD's with both 308 and 223 (thanks to help from forum members here!) with a best of 4.6SD for the Sierra 69gr. TMK's with an ES of 11. I was pretty stoked. My groupings however, weren't what I thought they'd be so I'll be doing some "proof" 10-round runs in two different charge weights. I know both guns can shoot. I've gotten less than an MOA on both @100 yards with factory match ammo and some of the rounds I've produced. I may avoid monolithics in the future as I get better results in fewer test rounds with standard bullets. Thoughts?

 

[Ape_Factory]

I did a test of the Hornady 70gr 223 monolithic bullets (#2281) and it was very clear cut in that the numbers steadily and evenly improved with each change in charge up until the final load. I go in .4gr. increments. There are two areas I don't feel like I have things under control; crimping and overall brass length prior to loading. I still don't feel 100% comfortable crimping. I use a separate crimping die but I can't really tell when something's been crimped "enough" (or at all) even with a magnifying glass. I don't see the physical indicators. With that said, my next set of test rounds, I'm not going to crimp to see if that'll tighten the group. Case length will be solved in two weeks when my Giraud arrives.

AR15 auto loader. Five different charges of H335 with PMC brass, CCI400 primers with a 2.250 COL, case headspace set at 1.455 and the brass at 1.750", sized by someone else. Five rounds per charge. Using a Magnetospeed to chronograph the rounds.

21.9gr. - 2461fps avg, SD=17.6, ES=47
22.3gr. - 2502fps avg, SD=17.2, ES=44
22.7gr. - 2559fps avg, SD=14.9, ES=37
23.1gr - 2607fps avg, SD=14.6, ES=34
23.5gr. - 2679fps avg, SD=11, ES=29

As I progressed, the groups closed up until the last round of five at 23.5gr which did not produce a tight group and I don't think any fiddling with COL will make it tighter than the 23.1gr. load, too spread out. 22.7 and 23.1 produced the two tightest groups with 23.1 having the slight edge. Mild pressure signs at 23.5gr., primers pretty flat but no cratering and no buddha belly at the case web. A couple had very faint extractor marks but it's not virgin brass. Nothing alarming but I don't think I want to go any higher.

So with the above, I'm thinking of reloading at 23.1 and varying the COL and bringing it out to 2.260, no crimp. I typically do 10 rounds at this point as proof the load works. I'll most likely do 10 rounds at 2.250 as well which is what Hornady recommends. Unfortunately Hornady does not have load data with this bullet and H335 in a service rifle.

I think ultimately, monolithic are just very sensitive to case prep and neck tension in particular. I've seemingly nailed down a .308 load with Hornday's GMX150gr. as well but I'm actually all out of those bullets! Go figure. Only took me 100 rounds to dial that one in.

As a new reloader, just thought it was nice how clear cut the test loads were despite the fact the lowest SD and ES load did not produce the tightest groups. That still befuddles me (I know, barrel harmonics!). I can probably achieve tighter groups with better technique and a better bipod so I do take that into consideration. I also tested out a load I think will work for Sierra TMK69's and that had a ten shot SD of 19.6 and an ES of 47 with a 2675fps average and no signs of over pressure. Fairly tight group too. Almost invariably, the lowest SD group does not produce the tightest rounds.