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1/25/2018 7:38:29 AM
Posted: 1/4/2006 11:40:01 AM EST
I want to build a 9mm AR for our local steel matches. I would like to use my existing lower with a CMC trigger. I understand the hammer may be different for the 9mm AR.

Second I would like a gas block without the front sight post. I've seen pictures of them on 9mm AR's. Do standard AR gas blocks work on 9mm AR barrels? Can I run a JP adj gas block?

Lastly I want to be able to run 9mm beta c mags (for those long field courses). So I suppose I would need the mag block conversion to colt mags right? Will modified sten and uzi mags run in this mag block as well?

Thanks in advance.
Link Posted: 1/4/2006 1:02:22 PM EST
Sorry but I am not fimilar with a "CMC trigger". If you notice on the top of a standard hammer there is a notch. If you use a hammer with a notch at the top with a standard 9 mm bolt, the notch will catch a groove just behind the opening in the bolt where the hammer hits the firing pin. So when the charging handle is pulled to load the chamber, the bolt will catch on the notch. That leaves you with the bolt inside the stock caught on the hammer.

The 9mm AR upper is blow back operated. The inerca of the bolt, buffer, and force of the spring holds the bolt in place during the igniction of the powder. So you don't need a gas block at all.

If you use a 9mm magwell adapter with a bolt catch link in the rear of the adapter and you use factory Colt , ProMag plastic mags, or a mag that have been modified to add the bolt hold open tab to the back of the follower, will activate the bolt catch in your lower. If you use a mag that doesn't have a tab in the back of the mag follower, or a magwell adapter that doesn't have the link, then your lower's bolt catch will not be activated.

I had a modified STeN mag. It would work with my Colt adapter. But without a loading tool, I could only get 1/2-2/3 of the rounds in the mag. If an Uzi mag is modified then it will work with the Colt system. The Beta C system is not designed with a hold open tab. Betaco now offers clear drum covers so you can see the rounds in the drums. Hope this was helpful.

Link Posted: 1/5/2006 10:49:43 AM EST
"CMC trigger" is probably the Chip McCormick one. If it has a notch at the top front like most AR triggers it won't work with an un-ramped 9mm bolt, but should work with a ramped bolt. If it doesn't have the notch it'll work.

Will modified sten and uzi mags run in this mag block as well?

Sten mags = Oly upper
Colt/Uzi/ProMag/Beta = Colt/RRA/ASA/BM? upper

Link Posted: 1/5/2006 11:31:00 AM EST
I don't remember if the CMC trigger has a notch. Couldn't you just remove the notch if it had one?
Link Posted: 1/5/2006 12:55:34 PM EST
I'm not sure how removing the notch would work. I suppose you could take the "back" of the notch down at the same angle so there's nothing to catch on, but wouldn't that be removing the part of the hammer that hits the firing pin?

DPMS makes regular AR hammers without the notch, which will work fine with the 9mms. They can be used with the JP trigger, which might be an acceptable substitute for the CMC. If you ramp the bolt you can use any hammer, and it smooths out the cocking cycle and is easier on the pins too.
Link Posted: 1/5/2006 10:31:29 PM EST
Ok I like the ramping the bolt idea. Is this a modification I do myself or can I buy a bolt that is already configured in such a manner.

Second what about muzzle breaks, comps, and flash suppressors. I tend to favor comps to help keep the sights on target. I do plan on using this rifle for 2 night matches so maybe a flash suppressor would help. I've only shot pistols and shotgun at night matches. With the pistol at least I had a hard time tracking the front sight due to muzzle flash. The shotgun was easier since the pattern was more forgiving. Would some sort of flash suppressor be a benfit for night shooting and using a dot sight or is mostly so you don't mess up your night vision gear (which I don't have btw)
Link Posted: 1/6/2006 3:02:50 AM EST
[Last Edit: 1/6/2006 3:36:56 AM EST by canon3825]
If you look at the bottom of a bolt carrier you will notice that there is a "ramp" from the firing pin to the bottom of the carrier. That is the surface that cocks the hammer. Since there is a "ramp" the hammmer is cocked gradually. On a 9mm bolt (since there is no gas system, the whole unit is the bolt), there is no "ramp". It is pretty much a 90 degree angle. A 9mm hammer is designed with a little different geometry to be easier on the hammer pin. The 90 degree angle was used by Colt to slow the rate of fire in a full auto submachine gun. With a complete bolt costing $150 and up depending on the brand, I wouldn't try to duplicate the angle of a rifle carrier with a Dremel tool. There are a couple of venders that can ramp the bolt for you. I had Joe Black at http://www.m60joe.com do two bolts for me. I also heard that Lee Minor at http://www.lrmfirearms.com/pages/507919/index.htm does bolt ramping. You can buy the a new Colt bolt already ramped with a tunsten weight instead of steel which SAW Sales calls "BOLT ASSY, 9MM SMG - IMPROVED MODIFIED COCKING ANGLE, TUNGSTEN WEIGHT" with a selling price of $395.

The question to ask yourself is how much effort do you want to do to switch from 9mm to 5.56? I have a Colt M16 RR. I'm not firmiliar with the semi auto 9mm hammer. If you ramp the bolt you don't have to change the hammer in your lower. To have someone ramp your bolt it could cost $50-100. A 9mm hammer for standard (.154") trigger pin is $15 from RRA + S&H. Maybe some else knows if a 9mm hammer will work with a 5.56 upper. If it doesn't then you have to change hammers when changing calibers.

For that matter the buffer. When I got my first 9mm converion upper, I was changing the buffer from an "H" buffer to the heavier 9mm buffer. The 9mm buffer was too heavy for my 5.56 uppers. The 9mm buffer slowed my 5.56 carrier so it would eject the spent round and strip a new round. But the carrier wouldn't go back quite far enough to cock the hammer. I bought a MGI rate reducing buffer. It makes all my caliber conversions run. It also smoothed out the recoil. Sight recovery with the MGI buffer was quicker even in semi.

Unless you buy a post ban barrel, everthing comes with a flash supressor if you buy a complete barrel. Since Colt was the one to come up with the 9mm set up they decided to have their 9mm barrels threaded to a different pitch so an armorer wouldn't thread a 5.56 NATO flash suppressor onto a 9mm barrel. The 5.56 has a smaller opening in the flash suppressor. The standard 5.56 thread is 1/2X28. Colt started using 1/2X36 for their 9mm barrel threads. Since Rock River Arms and Grarrison Manufacturing are Colt clones they use the same pitch. I'm sorry but I don't know what any other manufacurers use for a thread pitch. Since a 9X19 round has around 400 ft/lbs of energy and a 5.56X45 has around 1,100 ft/lbs of enery from a 16" barrels, muzzle rise should not be a big deal with the 9mm in semi auto.

One thing you might want to concider. Colt is the only manufacturer that I know of that makes a chromed lined barrel. Colt has "The" name, so you pay more. If you plan to shoot 9mm a lot (9X19 is much cheaper than .223 Remington) or you aren't so good about cleaning, I would recommend looking for a Colt barrel. If you are not going to use it much AND you are very good about cleaning and lubing then any one of the clones will be fine.

It is my understanding that a steel barrel in 5.56 is good for 8-10,000 rds. A chromed lined barrel is good for 20-25,000rds. 9mm moves at around 1/3 the velousity of 5.56. That would make for less wear. It is still a chunk of lead with some copper around it that slides on a surface of steel or chrome. The chrome is much harder. So it will last much longer. I think the bigger issue for most shooters is corrostion. I have to admit that a lot of the time it takes a while for me to clean my uppers after I have used them. Everything I have is chromed lined so I don't worry about it. Once you have a rust pit it will never go away. I'm not a big fan of Colt. I feel their stuff is over priced. They do make a good product but you pay "through the nose" for it. I would rather spend $250 for a barrel that I don't have to worry about than spend $175 on one that I know won't last half as long and I have to make sure it is lubed so it won't rust. This is my $.02 worth. Good luck with your build.

Link Posted: 1/6/2006 3:31:55 AM EST
[Last Edit: 1/6/2006 3:32:56 AM EST by joe1972]
Here is a picture of a ramped bolt compared to one that was not ramped!

The ramping gives the hammer twice the length of the normal stroke to reset, and thus is easy on the hammer, pins, ect...
Link Posted: 1/6/2006 1:22:15 PM EST

Originally Posted By joe1972:
Here is a picture of a ramped bolt compared to one that was not ramped!
This is the correct comparison pic -- copyrighted, so you have to click HERE

The painted pic above shows the ramp on the 9mm bolt to look like that as seen on the M16 (5.56) carrier

The ramping gives the hammer twice the length of the normal stroke to reset, and thus is easy on the hammer, pins, ect...

Take and weld the top of CMC hammer and profile it to look like the front/top of a M16 hammer, then your top swapping adventures will be nomore involved than simply swapping the top.

Click HERE also.
Link Posted: 1/7/2006 10:14:03 AM EST
[Last Edit: 1/7/2006 10:15:27 AM EST by AllCn1]
Here's a link to what the hammers look like.There is also some other good info there.

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