I am cutting a spare barrel down to 10.5" for my SBR lower. I have all tool ready, and even a die to re-thread the end.

But what should I use to open the gas port and how much should I open it up?

Thanks!

I don't know the answer to your question M4 but i got a couple of tips from my FAL experience.  If you have a chrome lined barrel then you'll want to use some sort of lube like cutting oil when  you drill, go slow and use an old cleaning rod or dowel in the barrel to keep from going too far.  If you're not careful the bit can bind in the chrome and break off then you have a problem!  With a FAL you just leave the sightpost out and fire a round resulting in a HSS projectile at 90 deg to the bullet.  THis of course wouldn't work as well with the AR.

A safe choice is to go up one wire gauge drill bit at a time, until you've got enough gas coming through to function reliably.  You can always go up another size at a time, but it's a lot more difficult to back up and try over if you went too far :)

Theres a thread i just ran across where they are discussing port sizes.

[url]http://www.ar15.com/forums/topic.html?b=3&f=66&t=165770[/url]

There is no substitute for experience. You might want to address your question to Kurt on the board discussion at
www. kurtskustomfirearms.citymax.com.

Where would we all be without Kurt's fine work!?

If the barrel is chrome lined, cut it to 11.5".  The extra inch of barrel will make a world of difference on the upper running.

If the barrel in not chrome lined, then cut it to 10.5" and have the chamber sidewalls reamed a bit oversized (polish the hell out of it).  This will allow the spent case to be extracted more smoothly then the standard chamber will allow.


[From one of "OldGuys" posts]

20" 0.093" Bushmaster and Colt
16" 0.063" Bushmaster Rep
14.5" 0.067" Bushmaster Rep
11.5" 0.076" old guy
10.5" 0.089"

Years ago in the now-defunct Machine Gun News, John Norrell wrote an article about drilling gas port sizes for shorties.  These are the numbers he gave at the time:

14" or longer barrel = .063"
11.5" barrel = .081"
10" = .093"

But according to a friend of mine who works at Picatinny Arsenal, original Colt engineering prints list .093" for all barrels including 20".

I've played with several Colt XM177 shorties, and I found that function with a bare muzzle was hit or miss.  The factory "moderator" does wonders for reliability by adding extra back pressure, so you might consider getting one of those or a suppressor if you have problems.  On one shorty, I could even see a difference in functioning whether or not it had a standard flash hider.

I wonder if someone (Kurt, maybe?) ever moved the gas block on a shorty 7" back from the muzzle under the handguards a la Bushmaster  Dissipator.  Now THAT would make a real nice shorty!!!!


[Edited to ficks speling misteaks.]

Thanks a lot Homo_Erectus!!

I am going to open it to the .093" What size drill bit might that be?

1/4"!

Closest is a #42, although you'd probably be ok with a 3/32".  Here's a handy [url=http://www.stosc.stsci.edu/instrument_lab/drill.html]drill size chart[/url] that will help you out.

No, that would be .25" Didnt pass third grade math did ya? [:D]

You just caught me off guard with a Subject that actually had any relevance to this site!

You can as easily have functioning problems from the gas port being too large.  M4, it would be advisable to start smaller, and work up.

With the gas port too large, the bolt carrier speed is excessive, it strikes the back of the receiver extension (aka buffer tube), and bounces forward too soon.  It may try to chamber a fresh round before the extracted brass has a chance to clear the ejection port.

Such a rifle will be felt to have excessive recoil, even to the point of a bruised shoulder.

As far as gas ports, bigger is not necessisarily better.

Simple procedure--

Start with a #60 o-ring from the hardware store, install it to the extractor spring

Starting with the existing hole, load 1 round to a mag, fire it, if it comes back firmly and locks bolt to rear,,, it's a GO..

If it does not come back enough to lock open, then open the port 1 size, try again...

Go get a # drill set, or buy a handfull of #bits, take a caliper and start a couple thousandths over the existing hole size, and collect the bits in .003 increments.


Easier to go with a 11.5" and a #45 bit.