358 mgp ar-15

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The brand new 358 MGP Stainless Steel .850 barrel arrived this afternoon. I will post a picture or two as I go along in the build.
Ed

[Rapier]

The bad thing about an AR-15 is regardless of how good it is, it still looks like any other AR, so bear with me on this. The camera was messed up while I was build in the gun, I know who is the culprit, me. So my pictures turned out to be the built gun.

Here is what I did. Got the barrel in, cleaned it out with Sweet Shooter. Then I put the top of the gas block together with the gas tube, turned a thread into the tube port itself with a 6x32 tap per instructions, turned the #6 allen head adjustment screw all the way in. Locked up the RRA Upper in the fiberglass vice block, lubed the receiver threads, and stuck the tightening ring on over the barrel using an AR-15 armorers wrench to cinch it up, I use a drill bit that just fits the gas tube holes to align the holes. Had to play with this one a bit, in and out until in went tight enough to line up. The J P Ent clamp on block requires a bottom plug regardless of how the confusing papers read, like you do not need one until you get to the end. So we got the sleve in properly and aligned all the holes and cinched the 8 screws down to hold the block. The bolt fell into a locked position from a drop, good to go.

Stuck the RRA Operator upper on the tightening ring and discovered quickly that the holes out at the end would line up on the adjustment screw, allowing me to adjust the gas without drilling holes or taking the for-end off.

I tried a 5 gr load of Bullseye under two sheets of toilet paper, it promptly blew a hole through the cardboard box I shot into and did not blow a bit of the brass out. Same old hard brass story, just like a 357 Harrett, get big, get hot or stay home. I decided to use jacketed bullets to fire form the brass and had two boxes of the old Sierra 170gr RN FMJ Silhouette 357 bullets. So drug out the 8 pound keg of 1680 and powdered their bottom real generously.
Dug out a junky Bushnell cheapo scope, I had no rings high enough to put a real scope on the rifle. But for fire forming the Bushnell will work Ok.
Ed

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These are a few more pictures. The #57 is supposed to be 357 and the gag is supposed to be Mag. The 6.8 Mags load up full capacity, 10 = 10 358s, 15 = 15 358s etc.
Ed

[Rapier]

These are the empty case bases in order, after fireforming with the adjustable gas block 32gr first row, first round at full close, no function
Turn screw out two turns brass left in action out of chamber, turn out 1/2 turn, brass out on ground, about 2 foot throw.
As I went up, I picked up all 5 rounds in a group, tarp on ground. When I got signs of scraping I turned screw in 1/4 to 1/2 turn notice only one or two slight signs, then none. Note 34, 34.5 and 35gr , the 34 was adjusted, 34.5 showed a light sign on one, adjusted, then at 1.5 turns out, 35 showed no signs of scraping, flat primers or enlarged primers. You could easily read the BR stamp on the primers.

When I say no scraping sign, that was after adjustment, which I think is the fair assesment of performance for the cartridge and the gas block.

The one missing primer was for measurment. .173-.1735 unfired and .173-.1735 after 35 grains of 1680. I poured them all in a tumbler and cleaned them up, except the pierced shoulder, it is a pin point hole burned through at about 33 gr. I just fireformed 51 cases to start because I did not know where or if they would show over pressure signs from 32 to 35 grains. Oh, I was using CCI Bench Rest SR primers in all Federal Brass. I want to return to the range with the repaired chronograph, a wire was loose on the 9 V Battery connection, with 3 loads 35, 35.5 and 36 to see if I get signs. One thing is the 1680 load is accurate in this barrel with the 170s but, it does not matter, because the bullets are no longer made. They were designed to function in a standard length 357 mag revolver cylinder. There is always a but, and this one is that the pre-foreformed cases at 35 gr 1680 were compressed, I suspect the bullet was touching the lands as they were not crimped and the bullet ogive is very short. I let one sit then put in back in the seater and there was resistance when the leaver was all the way down, so they grew due to the compression. Which is standard fare with H-110 & 296 unless you crimp them hard.

The 35gr load was 2470 FPS in this barrel but I want to remeasure the load due to the loose wire.

If the compression proves to be an issue with formed cases, it might be necessary to go a step faster to say 4227 to reduce the powder in the case and maintain the velocity.
Ed

[Rapier]

This project has progressed along. I finally needed to add weight to the rifle as it was putting a whooping on me, at 50 plus rounds per outing. So I stuck a heavy scope on it plus a Limbsaver clip on recoil pad for the carbine stock. Have the gas block almost turned off to stop scuffing. I made 2,500 fps with the 180gr spire points and 2,320 fps with the Rem round nose 200s. Then I found an old cardboard box of what really looks like the RCBS 200s W/O a gas check cut on a plain base. Stripped them of lube and powder coated them. That turned into a wow, at about 2,400 fps, no leading at all and sub .5 inch groups after about 30 rounds to get rid of the jacket residue. This was shot last weekend with 34 grains of AA-1680 over BR SR primers in Federal brass. I used a 357 Mag expander to open the mouth of the case to get the cast bullets to seat with the PC coating intact. The bullets are compression fit to the crimping groove.

What this AR has turned into is a 358 Winchester bolt gun performance in an AR-15 Carbine. The 16 inch barrel speaks with authority and with the polymer lower, it is a very light carry gun even with the added weight I had to stick on it.
Ed

[Rapier]

Took the 358 MGP rifle back to the farm range on Saturday. I actually ran the loads I had not run before across the chronograph. The 200gr Powder Coated bullets ran at 2,375 fps (fairly close to my estimated 2,400) and the 200gr FTX bullets ran at 2,369 but the FTX bullets were to long to work in the magazine so they will need a bit of tweaking.

Mike Plinkington, the MGP, sent me a note that he was playing with 9mm 115gr bullets and 296. He had gotten good enough accuracy at 2,815 fps to shoot coyotes with. So I went digging around in my bullet cabinets and found a 1,000 cardboard box about 2/3 full of Remington 125 JHP bulk bullets. I recut the feed ramps, polished them, and got the 125 to feed from a magazine without damage. Seems like if I can get good accuracy with them, at 2,800 fps they should be pretty explosive for varmints, unless the jacket strips off. Something to play with, using H-110 or #9.
Ed

[Rapier]

Since my last writing I have run tests on other bullets from 125gr Rem JHPs to the 225 Nosler Acu Bond. Some results:

125 JHP H-110 32 @ 2,760 fps .87 group
140 FTX H-4227 32.5 2,717 fps .75 group
170 Sierra Silhouette 1680 36 2,498 .036 group
180 Hornady Power Shot 1680 36 2,518 .86 group
200 FTX 1680 34 2,369 fps .52 group
200 RCBS Cast LBT Blue Lube 2015 12.5 Sub Sonic still working
225 Nosler AB 1680 32 2,147 fps .237 group *
250 Hornady JSP Subsonic still working

* this load tops the 358 Win in a bolt gun with a 24 inch barrel

Also since I wrote last I have a new 1-12 twist 358 MGP gun for heavy bullet subsonic work and have built a new suppressor on a Form 1 Corporation build. I just got the can finished and anodized the titanium can yesterday.

I also have a Sightmark Photon 4.6 Day / night digital scope and a 20 IR illuminator to put on it. Going to try yotes in the dark as soon as I get the subsonic ammo going.
Ed