Problem Child Build Spanked - .458 SOCOM SBR GTG

Mountain

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That 308 M5 was a quick and lazy assembly for me, so I thought about what else have I always wanted to eventually build. The stupid AWB going to the MA Senate made me decide to get off my ass, get a lower SBR'd, then start on the .458 I always wanted to build. I'd be pretty bummed if I procrastinated too long and had trouble sourcing parts or worse yet... no, not gonna go there.

I already have a .458 rifle and from time to time shoot the tar out of it. I reload Starline brass with 300 JHP's and 4198 powder. This load turns pumpkins basket ball size and smaller literally into sauce. I've mentioned this before, but on a range day with a buddy we shot through the same hole in the target at 50 yards for a couple mags full. When we retrieved the target we saw that we had blown a tunnel over 4 feet through the berm sand and were hitting the railroad ties supporting the back of the berm. Such a fun round and it can be loaded on the cheap once you have enough brass. Absolutely devastating terminal ballistics.

My plans are to go with a Tromix 10.5" barrel and bolt combo so I have the "original" guts. Going to dig through the parts pile and see what lowers I have to SBR. Already completed the fingerprint sheet but need to complete the rest and take a trip to the Mill for the proper markings. I have most everything else I need in the parts pile as well except for possibly a trigger that I really like. Probably will pick up a LaRue for that.

Here's what 300 grains looks like after hitting a 45 degree wet sand berm (shameless repost):

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Here are a couple of my typical loads next to what I think was a .223. The JHP's actually run very well and I never have feeding problems.

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I need to get tooled up to reload .458, what bullet are you using? Also any luck with subsonic rounds?
Mostly #4500 Hornady 300 gr JHP. Often I can find them for decent pricing. Haven't bought any in a long time because I stocked up. The tipped FTX or whatever they are cost more and don't really bring anything to the table IMHO. I'm going to try cast, I think 325's if I remember correctly- have not got around to running that mold. I'll use gas checks and hi-tek coating.

I have not tried subsonics yet. Can't have a can in Massachusetts plus subsonics are very heavy which = $$$$$$ to load factory projectiles or will consume a LOT of lead to cast. Might be fun to pop off a few rounds for fun and just to have a load developed, but for me they would be impractical. If I move to a free state I'd get the tax stamp and a can then load up some subsonics.
 
I've been neglecting my .458's, I have a second build that I haven't even shot. Last loads I made were LeHigh 300gr Match Solid's over 38 grains of CFK BLK, they worked wonderfully but I didn't crono them.
 
Haven't done any builds in a while so I just got a Tromix barrel/bolt in 450BM, was gonna go 375 SOCOM but there isn't much out there for ammo and parts whereas 450BM is a lot more available.
 
LOL, I think have at least bonked if not all out punched myself in the sausage on this one...

Granted it was something like 12 to 13 degrees on the day I took this project to the range, but for sure at least one issue was self inflicted. All of my AR's including match guns have been assembled by me plus I've built M1's, an M14, and an AK, so it's not my first rodeo but one would think so based on my first range day with this one. FTF's, one FTE, light primer strikes, and finally a slamfire ND! How's that for a clusterfluck?
[rofl]

This is a build with a Tromix barrel and bolt combo. BCG is Aero nitrided, other components are basic stuff with some salvaged from another AR that ran fine. It was lubed fairly wet and IIRC it could have been RemOil, which turns to gum at these temperatures. Possibly there was a metal chip or something binding the firing pin if it wasn't gummed up lube, as the light primer strikes were followed by the slam fire. I had cycled it empty and dry fired a couple times after the light strikes and it seemed to be OK so I tried another round. Surprise boom.

Having a very lightweight 458 go off unexpectedly is a good reminder why we always keep the business end pointed safely downrange. My 'ND' safely hit the berm.

When it did run, it was surprisingly accurate for a SBR. Also I tried to see if it would destroy a Pinty red dot and the Pinty held up. I have a lightweight minimalist carbine stock that was a bad idea for this and will be changed. This thing is so much more beastly to handle vs. my 458 rifle plus the short stock length let the charging handle bonk me in the nose.

Bolt had difficulty picking up rounds off the mag, so that's the FTF issue. I'm using the same mags as for my 458 rifle and on that they have been 100% reliable. I'll be checking my cases to see if they were the FTE problem. They have all run fine in my rifle (Radical upper) but it's possible this Tromix barrel has a slightly tighter chamber. I've probably fired ~ a couple thousand rounds of this load with zero issues until trying them in the SBR.

I'll post a pic from my phone later. I think I went too far in making such a lightweight build, as the recoil is pretty insane compared to any other AR that I have including the 458 rifle that is actually smooth shooting with a strong push but otherwise has manageable recoil.
 
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Haven't done any builds in a while so I just got a Tromix barrel/bolt in 450BM, was gonna go 375 SOCOM but there isn't much out there for ammo and parts whereas 450BM is a lot more available.
The same reason I picked 450 bushmaster over 458. Lack of readily available ammo and cost.
I have a lightweight minimalist carbine stock that was a bad idea for this and will be changed. This thing is so much more beastly to handle vs. my 458 rifle plus the short stock length let the charging handle bonk me in the nose.
I also found this out the hard way with my 450 bushmaster build.
Check out adaptive tactical. It's a bit chunky, but the pad at the end solves a good amount of the shoulder killer.
 
The same reason I picked 450 bushmaster over 458. Lack of readily available ammo and cost.

I also found this out the hard way with my 450 bushmaster build.
Check out adaptive tactical. It's a bit chunky, but the pad at the end solves a good amount of the shoulder killer.
Not really hurting the shoulder, it's just ridiculous for recoil. I tried a tighter grip on the M-lok hand guard, which did nothing to smooth out the recoil and made the hand guard more or less a cheese grater. The pad on that Adaptive Tactical stock does look pretty good. I have an Israeli made stock in the parts bin that I'll try since it's 'free'.
 
Not really hurting the shoulder, it's just ridiculous for recoil. I tried a tighter grip on the M-lok hand guard, which did nothing to smooth out the recoil and made the hand guard more or less a cheese grater. The pad on that Adaptive Tactical stock does look pretty good. I have an Israeli made stock in the parts bin that I'll try since it's 'free'.
ah I see.
Everyone has their own system for foregrips.
I made my own little system from fab defense and use it on every rifle
 
They found the 5.56 to be lacking in the Battle of Mogadishu (Blackhawk Down, etc.). Big, drugged up locals were taking hits but continuing to fight.

.458 SOCOM - Wikipedia.
Holy crap. Well that certainly must have solved the problem. I bet it makes a great short range hunting round for really big animals.
 
Holy crap. Well that certainly must have solved the problem. I bet it makes a great short range hunting round for really big animals.
It's a good round for dropping feral hogs. Terminal ballistics are essentially same as for .45-70, so it would be a good round for grizzlies too.
 
Well here's a pic of the Fail-O-Matic:

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Thus configured it weighs only 5.5 lbs, which I do NOT recommend LOL.

Pretty sure I have diagnosed the feeding issues. I had grabbed a charging handle from the parts pile and it is dragging badly on the bolt carrier.
 
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Went back to the parts bin and dug out a higher end ambi charging handle and a butt stock. Bolt carrier now rides smoothly back and forth and bolt closes with authority. Stock is Israeli made by FAB Defense. That little 5 round mag in the butt might only hold one or two .458's. I can probably shorten it a position or two but it was pinned for a prior build so I'll have to somehow yank out the pin. I'll give it a go this weekend and report back.

Look closely for the Barbi dress up accessory. [laugh]
 
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Went back to the parts bin and dug out a higher end ambi charging handle and a butt stock. Bolt carrier now rides smoothly back and forth and bolt closes with authority. Stock is Israeli made by FAB Defense. That little 5 round mag in the butt might only hold one or two .458's. I can probably shorten it a position or two but it was pinned for a prior build so I'll have to somehow yank out the pin. I'll give it a go this weekend and report back.

Look closely for the Barbi dress up accessory. [laugh]
look cool, but what was the incentive to sbr the .458?
did you calculate how much speed you lose there compared to 14.5,16, 18" barrels?

also, if it kicks - fab defence in same line has a shock absorber model.
 
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look cool, but what was the incentive to sbr the .458?
did you calculate how much speed you lose there compared to 14.5,16, 18" barrels?
Incentive? Compact and powerful. Eventually I'll cook up some heavy ~500 grain loads that don't really benefit from longer barrel lengths as the powder should be done in a shorter barrel. For what it's worth, 300 grain loads don't benefit from a barrel over 16", so the heavier loads with less powder won't be getting much if any benefit from the 16" barrel. For grins I can eventually check fps from my 16" rifle vs. the 10.5" SBR but it's not as if I care about hitting plates at 500 yards.
 
Incentive? Compact and powerful. Eventually I'll cook up some heavy ~500 grain loads that don't really benefit from longer barrel lengths as the powder should be done in a shorter barrel. For what it's worth, 300 grain loads don't benefit from a barrel over 16", so the heavier loads with less powder won't be getting much if any benefit from the 16" barrel. For grins I can eventually check fps from my 16" rifle vs. the 10.5" SBR but it's not as if I care about hitting plates at 500 yards.
did you chrono it? a 300gr in this caliber is supposed to be around 1900fps.
 
Figured out what was one of the issues. Bought a case gauge and function test dummies from Southern Ballistic Research, a vendor recommended by Teppo Jutsu / Tromix for .458 socom reloading supplies. Suspected that some of my ammo was out of spec, which was confirmed by a case gauge. I had reloaded and shot a few thousand rounds in the rifle with zero issues, but the batch I grabbed for the SBR were ever so slightly out of spec. The case head was barely above the gauge, but just that hair of excess was enough to prevent the bolt from going into battery consistently when I had tried to shoot them in the SBR.

The dummies functioned fine when fed from a magazine and their case head was slightly below the surface of the gauge. Given its large size and bottleneck, this cartridge takes a little oomph to resize in the press. Apparently my FL die had crept out a bit. Set it up to firmly bottom out on the shell holder, then ran several cases which checked out fine in the gauge and easily went into battery when fed into the bolt (you have to get the rim behind the extractor and hold the case down while it feeds into the chamber).

Should have had a case gauge all along but didn't need it until now. Here's the link to Southern Ballistic Research- SBR Ammunition | Tactical & Hunting Ammo | Sniper Rounds . Great service from them- good communication and quick shipping.

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Now I'll have to pull a bunch of bullets, resize, and reload. Dumb, avoidable mistake that I won't repeat. 🤪
 
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Figured out what was one of the issues. Bought a case gauge and function test dummies from Southern Ballistic Research, a vendor recommended by Teppo Jutsu / Tromix for .458 socom reloading supplies. Suspected that some of my ammo was out of spec, which was confirmed by a case gauge. I had reloaded and shot a few thousand rounds in the rifle with zero issues, but the batch I grabbed for the SBR were ever so slightly out of spec. The case head was barely above the gauge, but just that hair of excess was enough to prevent the bolt from going into battery consistently when I had tried to shoot them in the SBR.

The dummies functioned fine when fed from a magazine and their case head was slightly below the surface of the gauge. Given its large size and bottleneck, this cartridge takes a little oomph to resize in the press. Apparently my FL die had crept out a bit. Set it up to firmly bottom out on the shell holder, then ran several cases which checked out fine in the gauge and easily went into battery when fed into the bolt (you have to get the rim behind the extractor and hold the case down while it feeds into the chamber).

Should have had a case gauge all along but didn't need it until now. Here's the link to Southern Ballistic Research- SBR Ammunition | Tactical & Hunting Ammo | Sniper Rounds . Great service from them- good communication and quick shipping.

View attachment 849034

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Now I'll have to pull a bunch of bullets, resize, and reload. Dumb, avoidable mistake that I won't repeat. 🤪
I bought that case gauge the same day I bought my first 458 barrel lol, glad you got some of it figured out.
 
As mentioned, I should have bought a case gauge on day 1. After re-sizing the brass and reloading, all the cartridges fit the case gauge and readily dropped out of it. Problem solved- .458 SBR functioned perfectly at the range this evening.
 
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