Hello,

I just received the "7 inch" 9mm chrome-moly barrel I ordered from JSE. (It actually measures 8" in total length).

I put a 9mm cartridge into the chamber and measured that it stuck out about 0.123".

Then I measured the depth of the recessed bolt face on my new CMMG bolt, and it measured about 0.130".

This tells me that I will have about 0.007" gap between the bolt face and cartridge.

Can anyone tell me, is this too much of a gap?

(BTW the cartridge I'm using has a case length of 0.750"...is that about what it's supposed to be?)

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Edit: IIRC the bolt is not supposed to touch the barrel, and in this case it apparently will, so this means I will have to machine the end of the barrel and take maybe 0.0.010" off? Is that right?

My experience with 4 Colt's tells me that the bolt does in fact marr the finish on the barrel face.


Something that you may find useful is this post: http://www.ar15.com/forums/t_3_15/554541__ARCHIVED_THREAD____9mm___Checking_headspace_.html&page=1#i5298167

Thanks for the reply.

What SBR7_11 is implying in that thread agrees with what I've heard elsewhere; i.e., the bolt face doesn't quite touch the barrel.

Yet just like with your stated experience, I've heard still others say that the bolt usually does hit the barrel.

At this point I'm quite confused about this.

Maybe it's not critical, but if I have a choice, I think I'd rather have the bolt face a few thousandths away from the barrel.

Meaning that the cartridge being slammed into the chamber is what is going to stop a rather energetically moving bolt?  I'm waiting for some more info on this.

I really hadn't thought about any of this in a carbine; given a little time, I'll try to check mine.  Have you actually tried easing the bolt closed on an empty?

It sounds like some challenging measuring.

Moon

Now that you mention it, when I think about that aspect of it, it doesn't seem right.



Unfortunately I don't have any empty cases lying around here right now. I'll have to get some and try it.



So far I've just been using a digital caliper. Anyway, speaking of measuring, how does one measure the length of a 9mm AR barrel? JSE advertised this particular barrel as being a "7 inch" barrel; yet from end to end it's actually about 8 inches long.

I usually set a .010 to .012 gap on a blowback pistol cal buildas you are gonna get a little compression of the case when the bolt closes in cycling.......007 dosent sound bad though.....but I do question the accuracy of measurment with a caliper.....I make it a point to use a depth micrometer when headspacing a blowback gun.....

A barrel is measured from the recessed face of the bolt to the muzzle......sounds like you got some "free barrel"

Maybe someone should check the chamber depth and bolt face depth of some Colt parts to see how they do it.

ETA: in regards to chamber length and whether bolt face should touch barrel when a round is chambered

As regards barrel length, the deal is to drop a cleaning rod into an empty gun; allow it to seat against the boltface, and measure the distance between the boltface and the muzzle.  Complicating things is the flashider, which is not a permanent part.
Once you are in to NFA territory, I'm not sure how critical it is.
Moon

@Mad-Machinist:

Are you saying that when you set yours up, you had a 0.010 to 0.012 gap between the bolt face and the barrel; i.e., the bolt doesn't touch the barrel? (And yes, I agree about using the caliper, it's not the best way to make these measurements).


@ kevins_garage:

That would be good if someone could measure a Colt factory-set-up upper...and/or a RRA, CMMG or Oly, for the sake of comparison. I don't know how critical this is, but I really would like to know what the manufacturers are doing.


@ halfmoonclip:

That's the way I always thought barrel length was measured. But when I unboxed this barrel and measured it, I thought, hmmm...maybe there's a different measuring convention with AR barrels (since 7" was about what was sticking out from the upper receiver)? Apparently Mad-Machinist is right and I got some "free barrel".

I did a little experiment.

I put a cartridge case which measured about 0.748" long into the chamber and pushed the bolt onto it, and the extractor just barely grabbed the rim (the bolt was in contact with the barrel).

By "just barely", I mean it didn't immediately go over the rim; rather, there was a slight delay - apparently it was right on the edge and therefore unstable - and about half a second after the bolt made contact with the barrel, I heard the extractor slip over the rim.

Then, to simulate a "worst case", i.e., a cartride case that's a little shorter than 0.748", I took a piece of printer paper (with a measured thickness of about 0.0035"), and I cut a round hole in it just big enough so that the cartridge case could fit through it.

When I tried to close the bolt with the paper in between the bolt and the barrel faces, the extractor did not fully grab the rim.

This doesn't seem like a desirable situation to me.

I can envision tolerances stacking up the wrong way and adversely affecting the reliability of the gun. Maybe the chamber is cut is a little on the deep side, and the bolt face is recessed to the max, the case length is toward the short end of the tolerance, and firing pin is a little short (or the relevant bolt dimension a little long), etc., resulting in an occasional FTF or FTE; especially when the gun gets dirty or something.

Something is wrong.
You should have .128 for depth on the bolt.

You should have .135 +/-.005 on the case to barrel face measurement.

Yes....bolt dosen't touch the barrel.......010 to .012 isn't much at all if you look at the amoount of unsupported case you have in most blowback guns when you figure in the feed cone at the mouth of the chamber.......I've probably done 200 PPS43 barrels in the last couple of years using that spec and haven heard a complaint out of one yet..... and 7.62 Tok can be pretty stout pressure wise compaired to a 9mm.
Clip a round in the bolt under the extractor set the barrel on it and measure with a feeler guage.....as long as you aren't slamming the bolt into the breech face you should be good to go........and that is why I don't let the bolt slam forward on an empty chamber...I just ease it down.

Hope this helps

Can you try and get an accurate measurement from the breech face to the inside of the barrel nut retention shoulder on the barrel in question. You may be able to face the barrel just a hair and run it with no issues......while we talk about "headspace" in blowback guns......it is just used for lack of a better term......as long as the round can have the extractor engage and the bolt close with a minimal gap between the bolt face and breech face.....you are GTG.
I build a lot of 9mm blowback barrels...but I usually request the bolt that it is going to be used with be shipped to me....or dropped off for local guys......so I can actually cut the chamber to mate with the bolt correctly.

I was being facetious.  I'm not aware of any of the big manufacturers cutting their chambers so short that there is a gap between the bolt face and barrel face when a round is chambered.

I have a few other blowback rifles and the bolts in all of them bottom out against the barrel face when a round is chambered.  In a sense, they sort of slightly headspace off the extractor if the case length of the ammo being used is a lot shorter than the spec the chambers were cut to.  In my experience, most of the factory 9mm ammo I have checked has been several thousandths short of nominal spec for case length.

With that said, there are some boutique manufacturers and individuals that are doing some really weird shit...

I got into the habit of leaving a gap from doing a billion (or it seemed like at the time ) PPS43 barrels out of cut 1903 Springfield barrels The just seem to run better with less malfunctions  and as most blowback barrels are not hardened on the breech end.....the bolt continually slamming against the breech face can cause problems to both the breech and the bolt. As straight rimless pistol cartridges headspace on the case mouth......I noticed that when I used a headspace guage to set the cartridge protrusion to the point where I had maybe .003 or .004 between the bolt face and breech a natural swedging effect was causing the bolt to slam the breech and in some instances bounce off the breech face.......occasionally causing an out of battery discharge. I have a .40 S&W olympic arms barrel this effect is so noticable that it happens about every third round....even with an enidine hydraulic buffer.....by setting the gap between the bolt face and breech to around .010.....when the bolt is actually charging the round from the magazine while cycling.

The condition the OP is reporting is similiar to a customer attempted build DI .45 that was sent to me to trouble shoot......while the bolt won't close on a no go guage....and will fire when a round is hand cycled into the locked position......when feeding from a mag the throat of the chamber is a little wide and the swedging of the round by the violence of a firing cycle pushes the headspace shoulder of the case into the throat to the point of failure to fire and failures to extract.

I measured the bolt recess to be about 0.1285", or something like that.

And then I got my 12" dial caliper out and measured the barrel by itself, which was 8.000", and then I measured the barrel with a cartridge case inserted into the chamber, and the difference was about 0.118 or something like that.

So I think I need to face off the barrel and remove 0.012 to 0.017.

I'd just like to know what the factory specs are on the barrels and bolts (for uppers assembled by a big name manufacturer).

BTW I used to have a MAC10 (45 ACP) that fired from an open bolt. I seem to remember that it had a gap between the bolt face and the barrel (with a cartridge in the chamber), but I don't remember exactly how much.

I think 0.010 to 0.012 sounds good to me.

BTW have you ever worked on anything that fired from an open bolt? I know the cartridge has to stop moving before the bolt does or it won't fire, so I bet they generally have a significant gap.

Keep in mind that this isn't the same as an Uzi or a Mac, nor was it designed to fire from an open bolt...

What this boils down to, in my mind, is the question of how much of a "gap" between the bolt face and the barrel is acceptable? IOW, what's the downside? Is it a "safety" issue of some kind? Is it an issue of messing up the brass?

I think the comparison with an open bolt gun is relevant because, as I see it, at the instant the primer fires (and a few milliseconds thereafter), the cartridge doesn't know (or care) whether it's being fired from an open or closed bolt gun.

In any case, as Mad-Machinist or someone has already pointed out above, a gap of 0.010, for example, is small compared to the effective "gap" created by the bevel at the mouth of the chamber (i.e., an area where the cartridge case is not supported by the chamber wall).

Anyway, I sent emails to RRA and CMMG asking them about what specs they use in setting up their 9mm uppers. If I get a reply I'll post it here.

Have you fired the gun yet?

Why not load a mag, and fire it while checking the brass.
If the brass look good, load another mag and check for function of the gun.
If the brass look good, and it functions, load another mag and see if you have problems with a mag dump?
That would give you some good empirical evidence that your gun is fine.

I think you are over complicating the situation.

I understand what you're saying, and if I had all the parts I might put it together and try it. Unfortunately though, I'm still collecting parts.

The thing is, I'm familiar enough with mechanical things in general and guns in particular to realize that what I'm looking at here is a sub-optimal situation.

The gun may work ok as it is with the ammo I happen to have on hand right now, but what about with the next batch of ammo? If the cases happen to be a few thousandths shorter and/or the primers happen to be a little less sensitive, there's a good chance it won't work.

At the least, I want to know that if I happen to have cases that are, say, 0.744" (or whatever the minimum spec. is on the brass), that the extractor will still reliably grab the rim. I don't see that as being over complicated.

If my mini-lathe was not packed away in a box right now (in anticipation of moving) I would've already faced off the barrel and removed about 0.015".

I think next week I'm going to go see a good gunsmith/machinist and get his opinion.

I think you're right that I would only need to take a little bit off...like maybe 0.005 to 0.010 maximum.

Do you think I would need to take the same size cut off the shoulder as well, or isn't it that critical?