Back into AR's - Now not sure what to build, DMR, SPR, CQB, Retro, Flame Thrower.

RobertMcC

RobertMcC

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A year ago I did repairs on a friends AR and I was like this is the last one I'm working on ( was a Low Pro Gas Block, Midlength Rail ) and Building, So sent pops all my AR tools.

Ive had SPR 20", CQB 14.5, PDW 7.5/8". 9mm/223/5.56, had 22LR kits. Worked with 20" C7A1/A2, C8 FTHB.

Some I built/ Bought/Tweeked

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AR9.jpg



Well, a year later yet another project.

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Well ambi safety went right into the trash. Remember getting the C7A2 and was equipped with it and it digging into my trigger finger. Never really touched the midlength gas systems or modern style AR. But I still like A1/SP1 Uppers and FSB. Not sure what I want...
 
Rob, mil spec is 1.145" - 1.150" and commercial is 1.163" - 1.173" according to magpul. I didn't check my drawings, just googled it.
If the threads are larger in diameter than the tube it would be mil spec.
 
PEI ROB said:
Rob, mil spec is 1.145" - 1.150" and commercial is 1.163" - 1.173" according to magpul. I didn't check my drawings, just googled it.
If the threads are larger in diameter than the tube it would be mil spec.
Click to expand...

But this is a commercial, Angle back on the buffer. All I can do is try another tube. Unless the DA 556 lowers uses metric threads like the old CQ311 did.
 
With the angled back then definitely commercial but not sure I caught your point Rob. Thread size or tube size?

Mil Spec tubes are strong AND light. The tubes are thin but the threads are sharp profiled. After the threads, the tube diameter is reduced to make it lighter. The lower's threads are the same for mil spec and commercial.

Commercial tubes usually resist a dent better, are no stronger at the threads even with the larger diameter after the threads and are heavier. The threads are flattened which reduce the diameter but do not take any strength away. It does however promote an easier fit if made correctly. The tube inner diameter is supposed to be identical to mil spec so the strength is the same IF quality material is used. The thicker tube body obviously adds weight and of course will help reduce dents/crushing when quality aluminum treated properly is used.

If you are seeing 1.15x at the threads it may or may not be OK. 16 tpi with 12 thou taken off the tops is not a killer if the pitch diameter and profile are correct. A little on the high side but not a killer. That would bring the measurement down to 1.161 and perhaps what your calipers are actually reading?? I would judge it by measuring accurately but in this case I would advise for you to look at the profile. Are the tops or peaks of the threads flattened significantly? If so try the fit and if it feels normal to you, keep it. If they are not flattened out or the fit is loose, move on.
When you said 1.15x I was thinking tube diameter not threads. Didn't expect thread diameter to be that small.
 
PEI ROB said:
With the angled back then definitely commercial but not sure I caught your point Rob. Thread size or tube size?

Mil Spec tubes are strong AND light. The tubes are thin but the threads are sharp profiled. After the threads, the tube diameter is reduced to make it lighter. The lower's threads are the same for mil spec and commercial.

Commercial tubes usually resist a dent better, are no stronger at the threads even with the larger diameter after the threads and are heavier. The threads are flattened which reduce the diameter but do not take any strength away. It does however promote an easier fit if made correctly. The tube inner diameter is supposed to be identical to mil spec so the strength is the same IF quality material is used. The thicker tube body obviously adds weight and of course will help reduce dents/crushing when quality aluminum treated properly is used.

If you are seeing 1.15x at the threads it may or may not be OK. 16 tpi with 12 thou taken off the tops is not a killer if the pitch diameter and profile are correct. A little on the high side but not a killer. That would bring the measurement down to 1.161 and perhaps what your calipers are actually reading?? I would judge it by measuring accurately but in this case I would advise for you to look at the profile. Are the tops or peaks of the threads flattened significantly? If so try the fit and if it feels normal to you, keep it. If they are not flattened out or the fit is loose, move on.
When you said 1.15x I was thinking tube diameter not threads. Didn't expect thread diameter to be that small.
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BufferTubeComparison_zpsec9d6b28.jpg


I took the measurements like this. Measuring the commercial tube like shown ( Outside Dia ). Its only measuring 1.15X so it can be up to 10 thou off. But lower the buffer tube came off it was rock solid. It was replaced with a milspec tube.

I don't have another buffer tube to try. Both thread pitches are matching 16 TPI. So its not metric threaded lower ( Yes some Chinese lowers came in Metric weird thread )

So I'm trying to figure out WHATS out of spec. The buffer tube but came off a lower that was rock solid, or the lower. Almost the crap I dealt with the last AR I built that pissed me off not to want to build another.

Calipers are off but not major. 13mm wrench and nut ( i know that takes 13mm ) are coming up on the calipers are 13.3mm. The lower measured 1.13X which for a 1 3/16-16 should be 1.1875. So all my numbers are lower then I needed, their for should be loose. So throwing me off. To what out of spec, the lower or the tube...

Guess go get new calipers tomorrow.
 
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The inner thread diameter you would measure should read 1.134 maximum and 1.120 minimum diameter BUT that measurement isn't revealing any issue by itself.

The nut will make it rock solid. I wouldn't worry about it, unless of course you just want to know.
 
PEI ROB said:
The inner thread diameter you would measure should read 1.134 maximum and 1.120 minimum diameter BUT that measurement isn't revealing any issue by itself.

The nut will make it rock solid. I wouldn't worry about it, unless of course you just want to know.
Click to expand...

Well Ive never had one, that wiggled this bad w/o using the jam nut.
 
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