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