358 mgp ar-15

[Rapier]

I bought a complete 6.8 SPC upper on a whim. Then I bought 1,000 rounds of once fired brass from a salvage outfit, all Federal brass. Once I started testing the 6.8 loads I discovered that my 6.5 Grendel already had the territory covered by bullet weight, velocity and accuracy. The 6.8 upper was a duplication that I just did not need. So I started thinking about what I could do with the 6.8 upper. Then it hit me; As an old, 357 Harrett steel critter shooter, I still had my old Redding 357 Harrett dies. The Harrett cartridge is made by necking up a 30-30 to .357 then shortening the case to 1.75 and head spacing the case on the shoulder by fire forming in a crush fit chamber. The 6.8 SPC is actually a 30 Remington shortened to 1.635 then necked down to 270. The 30 Remington itself is actually a rimless 30-30. So If I expanded the 6.8's neck in stages, I could run a 6.8 into a 30 Cal sizing die then into a 357 Harrett sizing die and have pretty much a rimless 357 Harrett in an AR-15. Oh, now that would be one mean hombre in anyone's book, a very handy little AR-15 the size of an M-4 carbine with a lot of punch and easily capable of larger game from 200 to 300 yards. Plus it would be a very deadly wild hog or black bear gun on my farm.

So I started researching, reading my old notes and looking at my stores of bullets, powders, etc. In the process I found a group of mad 357 aficionados on the 6.8 forum's Alternative Calibers pages. They were already working on the same project and I found that Mike Pilkington of MGP Customs had been working on the project since 2008. MGP is Mike's initials. So I contacted Mike at mgpcustoms.com to pick his brain about what he had found out in his testing, you know, things that did not work, things that did work, things that should be discarded and things to keep. I was not going to plough the same ground twice, if it was not necessary.

I found that McGowen was going to make a run of .358 SS barrel so ordered a 358 AR barrel blank from them, they were about 3 months out. Then Mike at MGP offered me one of his 16.5 inch barrels already chambered, with the barrel extension properly ramped, gas port drilled and with a set of custom Redding dies for $100 more than the plane McGowen barrel, plus the MGP barrel had a 6 weeks lead time. The custom dies were worth more than the difference alone. I also found that Mike's 358 MGP cat on a Harrett cat, if you will, had half the neck length and more capacity than what I was going to end up with to start with. I was going to stop at the rim, without cutting it if I used a 357 Harrett reamer. So within two days of the McGowen order I cancelled the barrel from McGowen, sorry guys, and ordered the 358 MGP barrel w dies. The MGP cartridge case has an H2O capacity of about 39 grains. The 357 Harrett has a case capacity of about 41gr and the 357 Super Mag has a capacity of 36gr. The 358 MGP is functionally between the two, in an AR-15 rifle M-4 size configuration.

Speaking of capacity, it is important what brass you use to make the 358 MGP as every maker of brass for the 6.8 SPC makes a different capacity brass. It would not do to develop a max load with a larger capacity brass and then stick the load in brass that has two grains less capacity, for instance.

In the mean time I made up two rimless 357 Harrett dummy rounds using the Harrett dies, to see if they would function in the 6.8 mags. The 358 MGP uses the 6.8 mags at the magazine's full capacity, beautiful. Mike offered a suggestion which I followed, to remove the impressed forward rib in the mag, on each side of the magazine's side. The cut is down to the top of the mag latch's catch notch. The rib works against the bigger 357- 358 bullet diameter in the mag, forcing it in towards the center. When the impressed rib is removed the bullets feed right into the two ramps, one on each side of the chamber. Hum, things are looking good. The preferred mag is the PRI mag in 10 or 15 round styles, However in the 15 round version, the cult extends down past the mag well in the AR.

About this time I got some really good news, one of the guys out of Texas (he bought a barrel before me) and whom is testing loads using old silhouette powders, AA-1680 in this case (he is also an old Harrett shooter by the way) got a 2,510 fps read on a chronograph with a 180gr Speer bullet. Mike then went back and got a 2,466 fps read in his barrel with 1 grain more 1680 powder, without issue. That tells me that the general idea is a definite go and with careful work the 358 MGP can make it to at least 358 Winchester standard performances in a little AR-15. When you look up the 2,500 fps 358 Win loads for a 180gr bullet on the Hodgdon pages you find it to be a mid load between a start and max load. That is quite a feat for Mike and his cartridge.

I bought an RRA upper with the elevated solid rib for the scope and found a used RRA Operator free float handguard complete with mounting ring for about half price. I sold the 6.8 Upper and kept the BCG (the 358 MGP uses a standard 6.8 SPC bolt). I ordered the big .875 gas block (low profile) with the other goodies, rings, etc. I already had an RRA lower with an A-2 stock on my Grendel upper, so with an easy switch out, we can start running the 358 MGP brass when the barrel and dies get here.

I modified my first magazine and am now waiting for the barrel and dies to get here. Oh, Mike told me that his AR-15 weighs in at a trim 8 pounds which includes a snap on recoil pad. Yes the 358 MGP has some recoil. However the 358 MGP by all accounts is fun to shoot and hogs, coyotes and deer die very quickly when hit by the 358 bullet. Mike has a few pictures on his website one is a photo of a .32 inch five group, at 2,200 fps shot with his 358 MGP with a 200gr Hornady FTX over 23gr of H-Lil Gun at 100 yards. Thus you have accuracy and power potential in an AR-15 with a big 358 bullet.

After further study, I found that the hot loads with AA-1680 had the potential for gas scuffing on the case heads with some barrels or gas ports, so I ordered a different .875 gas block, an adjustable block that would fit inside the 1.750 ID of the Operator free float handguard, made by JP Enterprises that is .932 with a sleeve to .875. I can cut down on gas going into the action while still operating the bolt / action. I also found and bought two boxes of 180gr Speer bullets. I think I am going to use my old FMJ silhouette bullets and cast bullets to fireform the brass.

So far I have been making brass with two sets of dies, a 30-30 sizer and a 357 Harrett sizer die, a two step process. I might be able to even use the 357 Harrett dies to size the fired cases by setting the shoulder to the new cartridge after fire forming. The cases trim to a length of 1.63 with the OAL at 2.25 to 2.26 inches. My 358 MGP barrel should be in hand by the end of June.

The 358 MGP actually outperforms the 458 Socom at any longer distances, past 100 yards, as to retained energy and velocity due to the 357s higher BC and muzzle velocity. It also will fully penetrate 5/16 steel plate and may well penetrate .5 inch mild steel plate at 2,500 fps plus, which would make it an exceptional road block or check point weapon, when faced with thin skinned vehicles, similar to the old Remington #8 in 35 Remington. This would make it a contender for use by the military and the police.


This is the 358 MGP with a 180gr Speer on the left, and loaded with the old 200gr Remington HP on the right (first two L-R). You can see where the original 6.8 SPC shoulder was located on the left. The 358 MGP is a full length 6.8 SPC necked up to .357 then fireformed to the short necked configuration as shown.

Mike's polished stainless steel 16.5 inch barrels are .850 in diameter with a gas block area of .875. The groove diameter is .357 with the land diameter of .350 which allows for the use of .357 pistol bullets (with a round or truncated cone style nose) and .358 rifle bullets both. After much testing with 12, 14 and 16 twist barrels Mike settled on a 20 twist barrel for the best accuracy at the velocities his cartridge is pushing and the 100 yard 5 shot group of .32 inches attests to how accurate the 200s are in the 20 twist.



* 3rd from L This is L-R: 34, 34.5 and 35 gr of 1680 with the 180gr Speer. The base of the case was blackened with a magic marker to show any gas scuffing, none present.



Right picture. A .50 Cal hole in quarter inch thick steel pig made by the 180 Speer at 2350 fps. Bullet was .774 in diameter when recovered from 3 inches inside an ash log. The ash log was behind the pig. One of the traits of this round when shooting hogs is that it rarely is found in the hog, it goes right through them, even a shoulder shot, destroying both shoulders, will not stop the bullet.

If you want a 357 AR-15 with a bit more punch, this might be of interest. Contact Mike at mgpcustoms.com

I will follow along with the build and testing after I get my barrels in.
Ed

[Rapier]

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

[Rapier]

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

[Jack]

Rapier, you've really done a ton of work on that project!
Sounds like a very powerful package.