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Posted: 7/29/2013 10:51:44 AM EDT
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I spent about 9 months building this upper. Have not gotten to shoot it yet...ARG. I am having a hard time getting powder. When I shoot it I'll post the groups with the 300gr SMK loads.
338 Spectre UPPER: SBR 16” 1:6.5 Twist Barrel Gemtech 9mm Multimount with 3-lug adapter Paladin Machine 3-position Gas Block Rohg Arms, Black Viper Side Charging Upper Receiver w/6.8 SPC bolt Seekins SAR 12” Rail Forearm TROS 5/8”x24 3-Lug barrel adapter Magpul XTM Rail Covers CCOP 30mm 3x9x44mm Mil-dot Illuminated Scope (SCP-3944SI) GG&G 30mm 20moa Quick Throw Scope Mount Command Arms Front/Rear Backup sights GG&G Harris Bi-Pod Quick Throw Rail Mount Adapter Cere-Coat Barrel Coating by Tactical Toolbox in Toledo, Oregon LOWER: Bushmaster Lower Receiver JP single stage Fire Control system with oversized pins JP Silent Captured Spring Buffer system Magpul PRS Stock Accu-Shot PRS Rail-mount monopod. DPMS Panther Target grips Magpul aluminum trigger floor plate PRI 15 Round 6.8 SPC Magazine 
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I'm really curious how quiet it's going to be with the gas system shut off.
I currently have about 300 cases, that should keep me going for a while. I'm trying to get some Vihtavuori N110 powder for the 300gr SMK subsonic loads. I'm also going to make a 3-Lug compensator for running some super-sonic long range hunting loads. The gas block as 3 positions, "OFF, Suppressed, Unsuppressed". I have a 5 spring kit for my JP buffer, to tune it to the gas block, so it cycles in semi-auto reliably in both modes. |
This is my other toy that I originally bought the suppressor for. It's a Bobcat pistol converted on a Form 1. I have replaced the major trigger pack parts with HK parts, I also put on the Brugger and Thomet stock and forend rail system. The red dot has a ARMS quick-throw mount on it. I've loaded up some 147gr Hornady XTP JHP bullets in front of 6.3gr of Vihtavuori N320 powder. There is no gas blow back, no smoke from the muzzle, and the only thing you can hear is the action cycle. It can also put 5 rounds inside a quarter at 25 yards. It's a blast to shoot.
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| I don't have a can for mine yet but its in the works. I have taken 2 hogs with 250 gr smk at 1050 fps. It works but they did not tumble. I have a 310 gr cast mold on the way and I am hoping it will have a little more nock down on hogs. Even without a can its pretty quiet compared to my 6.8. I am also going to get a brass catcher. |
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UPDATE:
I got a load worked up for the .338 a couple weeks ago. I'm using 12 grains of IMR 4227 behind either the 300gr Sierra Matchking or the 300gr Lapua bullets. The Lapua bullet will seat further towards the rifling if they are loaded with the same OAL. I'm currently using a 2.26" OAL for both rounds. The Lapua bullet was a little more accurate, and was averaging just over 1 MOA at 100 yards. The SMK was just a little bigger group. With 12 grains of IMR4227 in the 16" barrel and the suppressor attached, I get 1075 fps with the gas system shut off (scary quiet!), and 1025 fps with the gas system in the "suppressed" position with the action cycling. I have recently picked up some 200gr Hornady V-Max bullets for working up a super-sonic hunting load, but have not gotten to loading any yet. The load data I have for that, shows the 200gr bullet going 1550 fps with 18gr of 4227. I'll post an update when I get it setup. I've had some issues with the 6.8spc bolt I got from DPMS. The extractor spring was not strong enough to pull the round from the chamber, so I had to add the extractor spring upgrade kit. The added spring pressure of the new kit also made it harder to get the extractor over the case rim, so I had to polish the extractor nose. I also had a problem with the bolt not fully rotating into position when the bolt carrier was fully closed. The gun would go "click", and there was no dent in the primer. I polished the lugs and the bearing surface on the bolt because it had a very rough finish. That seems to have taken care of the problem. Let me know if you all have any questions. |
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I didn't modify the extractor, I just replaced the spring with the Brownells upgrade kit by Buffer Technologies and polished the front of it. I am using factory SBR 338 Spectre brass. I have about 300 cases, so I probably won't be going down the 10mm mag case path.
The extractor works good now. If I have anymore problems.... I'm just going to buck up to the suds and buy the LWRC 6.8spc Advanced Combat Bolt ($125)and be done with it. I'm wishing now I would have known about it in the first place. |
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the .338spectre brass has the same extractor groove as 10mm Mag brass and IIRC there is a slight interference with that brass and the 6.8spc extractor. There is a picture somewhere that shows what is needed....IIRC it wasn't much work. That may be where some of your problems are coming from.
advntrjnky |
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I have not been able to find a picture of the modified extractor for the 10mm mag case. I did see a posting where someone said that a 7.62x39 extractor would work better. When I was having problems initially, I tried to swap out the extractor with one from a spare 5.56 bolt. They looked identical. I even saw some posts that said that they were the same. Both extractors had the same issue.
If someone has any input on the 7.62x39 extractor or maybe could send me the link on how to modify the extractor, I would be most grateful. |
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I just found this blurb from Marty on the 458 SOCOM forums:
Marty: If cases are hanging on the bolt (not being cleared) then the culprit is the extractor - the rim on .338 brass is ever so thicker than 6.8 and the corners of the extractor will "bite" into the brass. Take a small file and dehorn the corners of the hook on the extractor and you might be surprised how smooth it feeds and extracts/ejects. I'm going to do this modification this evening. |
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Quoted:
I just found this blurb from Marty on the 458 SOCOM forums: Marty: If cases are hanging on the bolt (not being cleared) then the culprit is the extractor - the rim on .338 brass is ever so thicker than 6.8 and the corners of the extractor will "bite" into the brass. Take a small file and dehorn the corners of the hook on the extractor and you might be surprised how smooth it feeds and extracts/ejects. I'm going to do this modification this evening. On a 'virgin' extractor, if you run your finger from the spring end to the hook, you will feel how surprisingly sharp the corners of the actual claw are. It makes sense, given that a small cutter created a perfectly square profile groove in a steel piece - the edges will tend to be sharp. Add to that the curvature and you have two nasty little corners that will tend to grab into the rim. That said, this typically only manifests as a failure to eject, not a failure to extract or a failure to fully go in battery. A 6.8 SPC bolt should not show either of those issues, the stronger extractor spring in this case might actually be working against you somewhat. I would suggest a polish of the chamber, plus dehorn of the extractor, and going back to the original extractor spring. Between those three things, see should feed and extract 'like buttah' |
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