So Why Is The Breach Face 89' 8" Travis

[shred]

I slapped a calibrated square on a breechface I had handy. Dead nuts 90 degrees.

That particular pistol has fired 95,000 rounds, and as the saying goes "I can count the malfunctions on one hand without using my thumb or three other fingers", so I'm thinking the builder knows what works, especially as how "rooney" guns are a lot harder to make run than run-of-the-mill 1911's.

So my question is "what does it matter?"

[Harmon]

shred, i guess STI pistols arent made off the 1953 springfield armory prints....SLACKERS!

Or dave dawson saw the design flaw and squared it up..

[Brazos Custom]

Shred, Harmon - STI uses an 89'0" degree angle relative to the frame rails. A little bit different than spec but every manufacturer has their own version of the blueprints - that's why most parts won't just drop in and you need them oversized. It is going to be hard to measure that dimension especially on a worn breechface. It is not a design flaw it is there on purpose. Also if you try to measure it referenced to the top of the slide you will be off because the top of the slide is not parallel to the rails - it tilts down slightly (the bore is parallel to the rails). Other manufacturers may vary slightly depending on the blueprint they are building to or the tolerances they hold.

[shred]

It's not an STI slide, though it is an STI frame. The top of the slide has been milled so I went off the disconnector rail and slide rails. All I know is it works, so I don't really care what angle it's at.

The "I know a secret, guess what it is" attitude irritates me (apologies if that wasn't intended, but that's what I got) since this forum is about sharing information (which Bob is great at, despite occasional frustration and forum contradictions from the know-it-alls), so I checked one to see if it was really important. I'll check some other ones someday..

[Flexmoney]

...this forum is about sharing information ...

Shred said it all right there. I've closed the other threads (we only need one thread to talk about this anyway).

As long as we stay on gun talk...and not people talk...we can keep this one open.

[Brazos Custom]

Just trying to get good info out there. Best way to measure is to set up the slide with the rails on a set of parallels and then use a dial indicator off the breechface as you move the table up and down. Just measured a Caspian slide that way and came up with 89.1 degrees about 7.5 thousandths per 1/2 inch.

[Brazos Custom]

As an aside and a little off topic it is important to realize that the breechface is not square when you are measuring to set up a gun. For instance if you are going to measure the distance from the first lug to the breechface in order to cut the hood you have to measure from the spot on the breechface where the hood will be when it is locked up, or when you are trying to see what the extractor hook to breechface clearance you need to measure at the spot on the breechface even with the middle of the extractor. Not crucial to be exactly in the right spot but if you are trying to get within 0.001 inch you need to be within 0.062 of the right spot vertically or you will be off by 0.001.

[shred]

Just trying to get good info out there. Best way to measure is to set up the slide with the rails on a set of parallels and then use a dial indicator off the breechface as you move the table up and down. Just measured a Caspian slide that way and came up with 89.1 degrees about 7.5 thousandths per 1/2 inch.

Cool.. I'll try that with some less worn slides.

If I get really bored, maybe I'll CNC the whole thing with a conductive touch probe and start a mail-in breechface-angle-measuring service... hmm ok, maybe not.

[cking]

Since I started this second round, thanks to everyone who contributed info. Sound theory of operation is essential to be able to diagnose problems, and be more than a dremel smith.

As far as who cares? Since this is the gunsmithing area, I think we all care!

Everytime I build a gun I learn more and it is better than the last. So being able to discuss this kinda of detail with real Smith's is a privilege.

Again everyone thanks for contributing!

[wsimpso1]

Ok, I went off and ran a bit of modeling and can report on that. And I still suspect that the breechface angle is what JMB and Colt and Ordinance worked out as making sense for their tolerances and everything with the barrel tilted muzzle down to achieve lockup. Different makers in different eras might work out other numbers.

One thing I did was I came up with an aproximate pressure curve that gave me about correct bullet velocities, and then calculated the resulting slide impulses, etc. And then, I worked out what the forces acting on the barrel through the link worked out to. JMB knew his stuff...

First off, springs are pretty inconsequential to what is going on. I got forces driving the slide rearwards running from about 2700 pounds (peak pressure) to about 160 pounds (as the bullet reaches the muzzle). Subtract out the 12 or so pounds that an 18 pound recoil spring has in battery and the 15 or so pounds that the mainspring contributes, and you still have 2670 on down to 130 pounds accelerating the slide...

Then I worked out the travel of the slide, the link movements, and forces involved in moving the link... The slide moves about 0.135" while the bullet is in the bore, and the first 0.083" is before the link reaches top dead center, or about 8 degrees, and the link is supposedly driving the barrel lugs about 0.003" deeper into the slide lugs. The mechanical advantage is pretty big because of the small angles. Even at the worst part, the friction of the lockup forces only requires about 30 pounds rearward on the slide to cause the lugs to slip. The bullet leaves the bore after another 5 degrees of link rotation, and 0.002" of barrel drop from TDC. Now think about link clearances - according to print, the two pin to hole clearances sum up to 0.001" to 0.085" , so theoretically, at least a third and as much as all of this up and down movement during the actual time the barrel is under pressure could happen without any forces in the link. We do still have to go farther than that to fully disengage the lugs, but then we have to finish the rocket engine effect of the powder gases draining from the bore. So, JMB had allowed for all of the firing forces, and movement, and draining the gases. Pretty cool. Do I think that the system is necessarily without movement between the barrel and slide during firing? Nope, but it could, depending upon how the clearances worked out.

Next, I did some calcs about sliding. Hmm, friction coeficients of smooth metals on each other go from 0.08 to about 0.20. Hmmm, a force normal of 1 pound would require only .08 to .20 pounds to make it slip, which works out to a force applied at about angles of 4.5 to 11.3 degrees to make it slip... Which is a whole hell of a lot higher than 0 deg 52 minutes (0.87 deg). Nope, I don't think that the breech face angle has much to do with feeding.

I am back where I started from. JMB & Co put that angle in there to make the breech face roughly square to the barrel, which is tilted down about that much. JMB put the barrel link 8 degrees past TDC to give the whole mechanism time to run the bullet out and drain powder pressure before the mechanism would unlock. The engagement and spring forces are so low compared to the powder forces that you can do simple conservation of momentum. And the stuff about the balanced force vector, UGH! Jerry Kuhnhausen writes a hell of a good book about gunsmithing these things, but his version of how it works still don't jibe with the physics and mechanical engineering I learned. And yes, the word is jibe (or gybe), you can look it up.

To get back to the original topic, I ended up sending the slide to EGW, and they said that it did not need welding. The wear at the top of the face from barrel impact and case rubbing had gotten my attention, and at my request, they are welding and dressing the face back to new specs, and I shall fit a new Scheumann barrel to that.

Billski

[Radical Precision Designs]

That sound you imagine hearing, is the clapping of my hands. Chalk one up to the "enthused" engineers. Especially those that re-invent the wheel in a daily basis, but keep in mind that it's still a wheel.

Now, back to shooting that baby untill you need some more welding ... or a new toy!

[Larry White]

So bottom line is you could have rebarrled it as it was, but you chose to reweld now instead of later?-----Larry

[shred]

How do you figure when the bullet leaves the barrel? Until that point, it's busy trying to pull the barrel out the front of the gun and locking up on the front of the barrel lugs.

[wsimpso1]

I just re-read my previous post, and the high side of the sum of the clearances between pins and the link is 0.0085", not 0.085" - purely a typo. Pity no one called me on it.

I did choose to weld the breach face, but not because it needed it now so much as because I figured that it had to be a little soft - the wear had occurred in less than 6000 rounds. I figured that the welded face would be more wear resistant and last a lot longer than the current condition face would.

As for reinventing the wheel, I did not. I just checked through the mechanics using the established dimensions, and found that it indeed works in the simple way that I thought that it had, complete with the slide and barrel engaged to each other well past the time of bullet exit (and well into drainage of powder gases). Simply put, the bullet is driven one way, and the slide/barrel in the other way. The Browning M5/Remington M11 shotgun, Remington M8 rifle, and the various Browning machine guns all use this simple method (taking advantage of Conservation of Momentum). The M1911 link is long enough that combined with going over center, the engagement between barrel and slide is not broken until the bullet is gone and much of the gas has drained from the barrel. In that time, there is plenty of energy stored in the slide to extract/eject the spent case, wind the springs, and allow a complete loading cycle.

As to how I estimated bullet exit, well, I know peak pressure and about what the shape of the curve looks like (guns and internal combustion engines have remarkably similar P-t curves). It varies with primers and powders, and even different cartridges all work the basic designs just fine, so the specifics of the pressure curve must not be too important. From the pressure curve, I calculated the force upon the bullet and its acceleration, and calculated bullet velocity and travel, all via numerical integration. I adjusted the scaling of the curve to give 850 ft/sec at 4.7 inches travel, and used that pressure curve to calculate forces upon the barrel/slide, and likewise performed the numerical integration to determine travel and velocity of the slide/barrel assembly. I did make the assumption that the bullet had about 100 pounds of drag on the bore, using up some of the applied force, but not much. I know from a similar setup at Remington Arms (Ilion) in the early 1980's that this method works. In rifled guns, some small drag (compared to the powder gas forces) is needed to make it match perfectly, while in smoothbores, you can neglect friction entirely. In any event, I stopped modeling slide/barrel response once the bullet cleared the muzzle. I did make a quick check of bullet vs powder momentum, and the powder gases are a relatively small recoil contributor (less than 10% here) in a low pressure, small powder charge gun such as a .45 ACP. Now if you load 9x19 to give Major, it will be a lot more, which Open Gun competitors use to run the brake/compensator. And going to high pressure, larger charge guns, like rifles, machine guns, and artillery, the gas exit makes up a much larger portion of the recoil impulse.

I do similar things for a living, and thought that by doing some simple modeling, we could all gain some insights into why these things work so well.

Billski

[tightloop]

My head hurts.....think I will just shoot it and not worry about the angle of the dangle...

[Singlestack]

My head hurts.....think I will just shoot it and not worry about the angle of the dangle...

+1

[wsimpso1]

"How about if I just shoot it?" I think that TGO said that... and it is a very good plan.