My personal inclination, if I ever got a AR15, would be for one in 7.62x39 for various reasons. So when I had the chance to help work up a practice load for one, well, yeah! Find out just what I think of it and assist in getting someone to the range.
He wanted a practice load for ranges out to 50 yards using the X-Treme plated bullet and H4198 powder. The bullet because A: they're much more available and less expensive than jacketed, and B: didn't want to use jacketed for general practice/plinking out to, say, 50 yards. Since the local indoor range rifle area runs to 30 yards, would work nicely there. That powder because he had a couple of pounds of it, and didn't want to chase something else down in the current situation.
The problem: you can't push these as fast as a jacketed bullet, and the minimum load shown for that powder and this bullet weight is 24.5 grains. For jacketed that shows 2190
feet per second; considering the max velocity recommended by X-Treme is
1500fps, that's a bit high. Especially since the softer plated bullets
are probably going faster than that.
Here's how it went:
This worked out nicely on the rifle in question: that does NOT mean it will in yours. I'm not recommending you do this, just relating what we found to work in this case. So if you try it and something untoward happens, don't yell at me.
We did try the 24.5 grains as a baseline; everything worked normally, no nasty fouling in the bore, but accuracy at 30 yards was so-so(~4". And it is driving the bullet way over what X-Treme recommends.
Reduced loads it is. We looked the information over and decided to try a big drop: loaded five rounds with 22.0 grains. Tried them, and liked them: better accuracy(about 2" at 30 yards), the action still cycled normally, and the cases looked normal. And 2.5 grains less per load works out to a lot of powder over time.
Further experimenting was called for, so loaded five rounds each in half-grain increments: 22.0, 21.5, 21.0, 20.5, 20.0, 19.5. All worked normally except the last: 19.5 grains would cycle the bolt just far enough to pick up the next round(three out of five), but not enough to lock the bolt back. It also had feeding problems. And the brass from those was really dirty: the pressure is not sufficient to cause the brass to fully expand and seal the chamber, so a lot of fouling in the chamber and on the cases. That one's out, even if it had had the best accuracy.
20.5 showed more fouling on the brass than full-power loads but MUCH less than 19.5, full functioning. About two inches at 30 yards.
21.0, 21.5 and 22.0 all gave full function of the action and normal-looking cases, groups around 2-3" at 30 yards off a so-so rest, and no sign of any nastiness in the bore. Since all seemed to give same-size groups, decided on 21.0 as the practice load. Reasons:
This is primarily a plinking/indoor range load, so higher velocity not necessary or needed, especially with these bullets.
If 21.0 works as well as 22.0, why use 22.0?
This was assuming it gave decent groups at the 50 yard distance; if it can't reliably hit a coke can at that range, well.
Occasionally, it was reported that the 21.0 load didn't quite cycle fully, so wound up going with 22.0 grains. Zero problems, and good accuracy.
Borrowed from this post, finally had the chance to run these over the Chrony, and the 22.0 grain load averaged 1839 feet per second; higher than X-Treme recommends for these bullets. However, no problem with copper fouling, no signs of bullet wobble or jacket-stripping, and good accuracy.
Yesterday had a chance to hit the outdoor range, which gave us 50 and 100 yards to try. Shooting from a solid rest, this load gave 2.5 to 2.75" groups at 50. Quite satisfactory. I did set up the Chrony to check velocity, but I'm not going to swear by the numbers. It was nasty windy and gusty, on the level I had to use a chair to block/support the tripod to keep it from blowing over, and some cloud around. I know light conditions can affect the readings, but opinions seem to vary about wind; in any case, the readings jumped enough that I don't trust them. Get another chance in the future on a calmer day I'd like to try them again. By 'jumped' I mean from the 1550fps range on some to 1860fps on others.
No, we didn't try them at 100. With the lower velocity and that wind decided wasn't worth it. Did try the rifle with some PMC ball and some handloads using the Hornady 123-grain softpoint; it liked both, when either of us did our part got sub-2" groups, which under the conditions I thought quite acceptable.
So if I wound up with a AR15 I know what I'd get it in. And I know the X-Treme bullets will do nicely for practice.