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Why 1/2x28 ?


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How come 1/2x28 is the "standard" thread for 9mm comps?

With so little material left, 

I feel like it'll snap right of, if I drop the gun...

And if I'll pin and weld on a comp, how close to the bore do I dare to drill?

Edited by grottulf
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  • grottulf changed the title to Why 1/2x28 ?
1 hour ago, grottulf said:

How come 1/2x28 is the "standard" thread for 9mm comps?

With so little material left, 

I feel like it'll snap right of, if I drop the gun...

And if I'll pin and weld on a comp, how close to the bore do I dare to drill?

My guess would be so you can’t put on a wrong comp and blow the gun apart

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39 minutes ago, Eric802 said:

I thought 1/2-36 was more standard for 9mm, and 1/2-28 for .223/5.56.  As noted, 1/2-28 is how AR barrels are normally threaded. My FM9 is 1/2-36.

Correct I got it backwards. It's still to keep from mismatching calibers

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3 hours ago, Sarge said:

Correct I got it backwards. It's still to keep from mismatching calibers

Lessons learned the hard way are lessons that are learned for life. If an individual puts a .223 comp on a 9mm barrel, they'll only do it once. 

 

What's actually annoying is there's two "standards" for PCC barrels. 

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Ok. Most 9mm comps I've seen has been 1/2-28. 

But nevermind the pitch, it's the choice of diameter I think is strange...

 

The thickness if the barrel will be under 0.06" with a 1/2" comp!

 

The comp itself will strengthen the barrel, but there will be a substantial stress riser between the barrel and the comp.

 

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6 hours ago, grottulf said:

The comp itself will strengthen the barrel, but there will be a substantial stress riser between the barrel and the comp.

 

And if you go to a 3-gun match you will see competitors actually throwing their ARs and PCCs into barrels comp first.  They survive.

 

FWIW, the threads on my PCC barrel are 1/2x28.

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I seem to recall Taccom saying not to use a crush washer to time their comps because you could damage the barrel due to it being thin

Also 9mm barrels used to be commonly  threaded 1/2x36, same as the 7.62x39 which made it harder to do stupid things

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15 minutes ago, rishii said:

I seem to recall Taccom saying not to use a crush washer to time their comps because you could damage the barrel due to it being thin

Also 9mm barrels used to be commonly  threaded 1/2x36, same as the 7.62x39 which made it harder to do stupid things

You can snap the end of the barrel off if you use a crush washer and need to go more than 1/4-1/3 turn to time the comp......1/2-28 on 9mm is THE dumbest thing ever!!! It should be 5/8-24.....change my mind  :)

 

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5/8-24 is used for .30 cal.  Some of those may be too tight for 9mm.  I agree, 1/2-28 is dumb for 9mm.  The barrels I've seen with that come with that thread have a big warning.  But that doesn't eliminate the chance for accidents.  For the 1/2 thread 9mm barrels with pinned and welded comps, I cut a horizontal groove at the bottom, then lay the pin in it and weld up the groove, which makes it permanent.  If you put a pin in vertically, it's probably not going to be set deep enough in the barrel to make it secure.

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My recent experience (last two years} of building AR9's shows that until very recently, 9mm AR barrels were more often than not 1/2 X 36 but recently I find there are more barrels that are available being  threaded 1/2 X 28 and I think that's a mistake......

 

I think that 1/2 X 36 was a smart idea to keep the clueless idiots from putting their AR15 .223 muzzle brakes on a barrel that will blow up the gun (and possibly hurt themselves and others) on the first shot............

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4 hours ago, TRUBL said:

You can snap the end of the barrel off if you use a crush washer and need to go more than 1/4-1/3 turn to time the comp......1/2-28 on 9mm is THE dumbest thing ever!!! It should be 5/8-24.....change my mind  :)

 

I don't care what it is, all I want is for there to be one standard for a specific caliber barrel.

 

1/2" × 28 for .22 & .223 barrels

5/8" × 24 for. 30 cal & 9mm

And whatever it is now for the less commonly seen calibers

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5/8-18 is standard fine thread and would seem to be a good choice for 9mm.  Regarding 1/2" threads on a 9mm barrel, at the base of the threads the barrel od is .450.  So that gives you .095 barrel thickness at the thinnest point (assuming SAAMI spec .355 groove diameter).

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36 minutes ago, ltdmstr said:

5/8-18 is standard fine thread and would seem to be a good choice for 9mm.  Regarding 1/2" threads on a 9mm barrel, at the base of the threads the barrel od is .450.  So that gives you .095 barrel thickness at the thinnest point (assuming SAAMI spec .355 groove diameter).

Actually it gives you .0475 barrel wall thickness. 

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20 hours ago, ltdmstr said:

5/8-24 is used for .30 cal.  Some of those may be too tight for 9mm.  I agree, 1/2-28 is dumb for 9mm.  The barrels I've seen with that come with that thread have a big warning.  But that doesn't eliminate the chance for accidents.  For the 1/2 thread 9mm barrels with pinned and welded comps, I cut a horizontal groove at the bottom, then lay the pin in it and weld up the groove, which makes it permanent.  If you put a pin in vertically, it's probably not going to be set deep enough in the barrel to make it secure.

Thanks for the advice, but I'm not sure I understood exactly how you did it.

Do you happen to have a have pic of the pin and weld?

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10 hours ago, grottulf said:

Thanks for the advice, but I'm not sure I understood exactly how you did it.

Do you happen to have a have pic of the pin and weld?

 

Not at the moment, but I'm probably going to do one next week.  I'll try to remember to take some photos.

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