 K2unit 
Member
Joined: Dec 2006  FL, USA
 Posts: 110 Feedback: 100% (1) |
Posted: 7/30/2012 4:38:42 PM
THE IMAGE ABOVE IS A PAID ADVERTISEMENT What is the Barrel/Trunion thread for an American SIG 551A1 & 556? NOT the flash hider thread which I believe is 1/2x28 (AR15 Standard) I want to build a Swiss Style SBR 552 (10” Barrel) from a SIG 551A1. I just picked up a true Swiss SIG 552 cut receiver parts kit and measured the thread and it looks like 22 x1.5mm. Rather than cut the barrel and re-work the gas block on a 551a1, I thought it would be easier to just replace the Barrel & Gas block from the Swiss 552. BUT>>>>>> I just recently heard that the American 551A1 & 556 has a different barrel/ trunion thread! Can anybody confirm the different thread and do you know what the American version thread might be??? Thanx K2 |
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Posted: 7/30/2012 4:46:25 PM
[Last Edit: 7/30/2012 5:06:27 PM by JoshNC]
I don't remember the thread on the US 556/551A1, but it is different than the 552. None of the 55x smiths I've spoken to were willing to try to shoehorn the Swiss 552 barrel into the 556 reciever. I know of one gent who destroyed his reciever trying to do so.
You are better off mod'ing the 556 barrel to 552 spec. If you are a machinist it should not be too difficult. The 552 gas block can be used. My 556/552 SBR was built by Frank Hatten at AMA. He mod'd the barrel to 552 spec. It is an awesome rifle. But I had mine built before the 556 pistol or SBR were available. For the cost, I think buying either a factory SBR, or registering a pistol as a SBR is the way to go. Then add the lower from the 552 kit, weld on diopters or a steel Swiss optic rail, refinish and be done. The factory 552 barrel is actually 8.9 inches from breech face to the end of the muzzle threads. The 556 pistol/SBR (including the 551-A1) barrel is 10 something inches. |
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Posted: 7/30/2012 5:51:42 PM
Still would like to know what is the 556 & 551A1 Thread.
would it be easier to put a 556 Pistol Upper on the 552 lower…even if the barrel is 2 inches longer, still SBR! Can you put the SIG side folder stock on the 556 pistol lower? Or is the rear trunnion permanently dedicated for pistols only? Do they make 551A1 pistols? Is the upper receiver on the SWAT model the same as the standard upper? (Just remove the quad rail?) |
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Posted: 7/30/2012 8:08:45 PM
7/8-20 UNEF is what the US trunion is.
Using a 556 Pistol to make an SBR is going to be much easier/cheaper than rewelding a Swiss trunion or buying a Swiss 55x receiver. Some people make 551-A1 pistols. 
You can buy a complete 551-A1 lower for about $350. P556's still come from the factory on ER lowers. PM me your email address and I'll send you the drawings I have for 556 measurements. |
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Posted: 7/30/2012 9:07:06 PM
I first put an A1 on my pistol to make my SBR...it was really easy...
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Posted: 7/31/2012 12:44:51 PM
[Last Edit: 7/31/2012 12:48:25 PM by JoshNC]
Originally Posted By K2unit:
Still would like to know what is the 556 & 551A1 Thread. would it be easier to put a 556 Pistol Upper on the 552 lower…even if the barrel is 2 inches longer, still SBR! Can you put the SIG side folder stock on the 556 pistol lower? Or is the rear trunnion permanently dedicated for pistols only? Do they make 551A1 pistols? Is the upper receiver on the SWAT model the same as the standard upper? (Just remove the quad rail?) If you must have a true 552, I recommend finding one of the semiauto title-1 (not SBR) 552-2SP previously sold by Midwest Distributors. There are around 80 total of these and one was recently posted on Sturmgewehr (I don't have a link) for around $5k. Once you factor in the mods to bring a 556 to 552 specs, refinish, cost of 552 parts kit, etc you will likely have well over $4k invested. Unless you are able to do the work yourself and are just looking for an interesting project. All 556 uppers will accept all Swiss 55x lowers as well as all generational iterations of 556/551A1 lowers. The pistol lowers are only capable of accepting AR15/M4 Carbine type butstocks, with the rear tang being dimensionally different from the Classic and 551A1 lowers. Pistol lowers do not accept Swiss/Swiss pattern folding stocks. Yes, Swat model has same upper as standard, just with addition of railed fore end. |
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Posted: 7/31/2012 3:57:46 PM
Just confirming,
A 551a1 lower OR a Swiss 552 Lower will both drop on a 556 Pistol upper receiver? Correct? Thanx K2 |
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Posted: 7/31/2012 4:07:10 PM
Originally Posted By K2unit:
Just confirming, A 551a1 lower OR a Swiss 552 Lower will both drop on a 556 Pistol upper receiver? Correct? Thanx K2 Yes |
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Posted: 8/7/2012 7:59:37 PM
Have they come out with a 551a1 pistol yet?
Thanx! |
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Posted: 8/8/2012 12:05:22 PM
Originally Posted By K2unit:
Have they come out with a 551a1 pistol yet? Thanx! How about a 551A1 Patrol!!  |
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