.38 amu chamber drawing

I'm looking for a dimensional drawing of the .38 amu barrel, either the Colt or S&W manufactured ones. I have a pristine S&W barrel that I'm having a six-inch long slide Colt built around. Problem is that the barrel is so pristine that it needs to be finish reamed.  Thanks Much

Greetings,

Here is the CIP drawing. No SAAMI that I am aware of.

https://bobp.cip-bobp.org/uploads/tdcc/tab-iv/tabivcal-en-page59.pdf

Cheers,

Dave

Colt38amu wrote:Problem is that the barrel is so pristine that it needs to be finish reamed.  

That’s not a problem, that’s what you want to get the correct headspace. 38 AMU cartridge headspaces of the case mouth. Just sounds like unfit barrel. You need to make sure that the finish reamer diameter is larger than rough chamber diameter. In other words, you want your finish reamer to cut the entire chamber. Have you dropped a cartridge into chamber? Does it fully or partially chamber? I’ve seen fat chambers that new finish reamer doesn’t contact the entire rough finished chamber. Also depending on headspace after fitting the barrel it’s possible that you will not finish ream. Just use a throater for proper lead angle. Can you provide two dimensions? 1) headspace unfit? 2) Dimension from rear of hood to the first upper barrel lug?
Thanks
Jon

Greetings,

The original 38 AMU headspaces on the rim, like a 38 Special.

It is possible to have a custom reamer ground to headspace on the case mouth. You would end up with a "wildcat" chamber. The 38 Super went this route.

Look closely at the CIP drawings. There is no shoulder to headspace against the case mouth.

There is also a flagnote 4 that says Headspace on rim.

Last, look at angle/dimension "i". It is 1 degree 30 minutes. This leaves a loooong leade that used to foul up badly with cast Lead bullets.

Are your barrels actually marked 38 AMU? I would love to see some images of the markings. Especially the proof/vendor marks on the sides of the lower locking lugs.

I did not know that S&W made 38 AMU barrels.

Cheers,

Dave

The barrel is at my gunsmith right now. I acquired it in a lot of .38 amu brass that I bought from a gentleman who was liquidating his grandfather's estate. According to him his grandfather was an armorer at Fort Benning back in the 50's and 60's. I have the Jenkins book on Military match pistols and they have a drawing of the S&W MTU barrel. According to Jenkins S&W provided the barrels in .45 ACP and .38 special, five-inch length. The .38 special barrel was supposed to be a drop-in replacement for the .38 amu barrel after the Military ordered all .38 amus in inventory to be converted to .38 special. The barrel I have is marked S&W MTU with NO caliber designation, (unlike) the Jenkins drawing). A .38 special case will not chamber in it, but a .38 amu will, the rim on the .38 special runs into the barrel hood. I don't know enough about it to say what it is or isn't, simply assumed that S&W knocked out barrels in .38 amu. Will post pics as soon as I can.

Jon,

Im building the amu.  Just went and took some measurements.the barrel hood to lug length is 1.327, slide breechface to lug length is 1.315. The hood to chamber mouth is 1.215 currently (some issues with measurement, but close). So after taking 0.012 off the hood, there will still be about 0.045 from the cartridge mouth to the chamber mouth which seems pretty long to me...

That’s a very deep chamber measurement from hood to chamber end. I just measured a new Colt 38 spl Kit Gun barrel. I’m coming up with approximately 0.012” from case mouth to end of chamber. Also measured another 38 Kit Gun barrel I just fit to pistol. Gap is 0.015. 
Most welded up and refit Colt stock barrels have 0.020” gap. NM 45 barrels are close with 0.015-0.020”. MTU 45 barrels are the same.
38 spl is headspacing off the case rim. The 38 AMU and 45’s are actually headspacing off the extractor on these deep headspace chambers.
My 38 AMU pistol was converted to 38 spl by Jim Clark in early 1960’s. The chamber was throated using throater reamer. You can barely make out the end of what used to be the chamber end. Chamber now is more like a long freebore into an extremely long lead angle into the rifling. I’ve never shot it so do not know the accuracy of the chamber/barrel. I have seen other older 38 spl barrels throated like this. I have a 38 spl S&W MTU barrel that I throated similar to this with freebore and 1.5 degree lead angle. I’m currently loosening that pistol to function factory Winchester 148gr Super Match ammunition. Should know soon how well it shoots.
So as far as your barrel is concerned I’m thinking that the cartridge case will have to offer some support/alignment of bullet as it enters the rifling/lead 0.045” down the road so to speak. Should be interesting.
Jon