0.005” makes the difference

Nearly a year and half ago I started having problems with my STI DVC. Sometimes the trigger pull would be MUCH greater than others. At times it would be so great that I could barely get it to fire. And it only happened at matches! On the next stage it might be just fine. It would never do that while in practice or when I was drying firing it at the bench at home.

Then, finally, three weeks ago, Ry and I were in the training bay at West Coast Armory and it did it again. I dropped the magazine, ejected the round in the chamber and tried dry firing it several times. It was just fine.

Okay. What gives?

I put the magazine back in and chambered a round. Impossible trigger pull. I dropped the magazine, ejected the round, and dry fired again. Just fine.

Magazine in and dry fire? Nope. It was a heavier trigger pull than I could manage.

I pulled on the trigger as I slowly removed the magazine. CLICK!

It’s the magazine! How in the world does the magazine affect the trigger pull? I tried it with other magazines. Three out of my eight magazines had the problem. Visual inspection did not reveal anything different about them.

This explains why it only happened in matches. I almost always use different ammo in practice than at matches (the indoor ranges where I practice require copper jacketed bullets and I shoot polymer coated bullets at matches) and the magazines with some left over match ammo are not used in practice. It finally happened that I removed the match ammo from the proper magazine and used that magazine in practice.

A day or two later I had the time to diagnose the problem with the magazines. I did some measurements and found the “bad” magazines were about 0.005” longer than the “good” magazines at the spot where the trigger bow goes around the magazine:


If the magazine was 1.366” or less everything was fine. The “bad” magazines were in the range of 1.367” to 1.371”. The 1.367” magazine had a noticeably harder trigger pull but not so much that it was much more than “odd”. And even with the 1.371” magazine if the trigger were pulled off to one side or the other rather than straight back then it would fire much easier.

I suspect the problem is really with the trigger bow rather than the magazines. If the magazine easily fits in the magazine well the gun should work. But putting the magazines in the vise with a couple blocks of wood and squeezing them 0.005” seemed less risky to me than messing with the trigger bow. My gun now works just fine with all the magazines.

I really should get the STI Trigger Stirrup Die from Brownells for the proper fix.

Man, that was a perplexing problem for such a long time.


10 thoughts on “0.005” makes the difference

  1. I had this problem with my Colt back in the early 80’s. In my case, it dragged on reset and often wouldn’t reset. Also only at matches. 😈. Replace the trigger with one with better shaped bow.

  2. I’ve noticed the same thing with my Norinco 1911. I bought it back in the early 90’s before the Clinton ban on the Chinese imports, and used mostly GI-surplus magazines through it. It always worked just fine.

    A year later I bought a used Springfield that I customized for bullseye competition (adjustable rear sight, beveled magazine well) and started using Chip McCormick “PowerMags” (I also had custom hand-loads with 200-grain SWC’s over 3.0 grains of Bullseye – they just barely went “pop” and I needed a low-powered recoil spring). I kept shooting competition until my eyes went downhill.

    Fast-forward a bunch of years and now I’ve got several other 1911-style .45’s, all of which feed from those higher-end magazines. I hauled the Norinco out of the gun-safe and took it to the range the other month just for old-time’s sake. Loaded up a magazine, inserted it (with no problem)…and the trigger wouldn’t move. Dropped the mag, cleared the round, and let the slide run forward empty and dry-fired it with no problem. Hmm. So I slapped the magazine back in with the ejected round back on top, chambered it, and tried to fire. Once again the trigger wouldn’t move. This time I dumped the mag, but didn’t clear the pistol, and pulled the trigger, which worked just fine.

    I happened to have included a ziplock with a half-dozen old GI-surplus mags, so I loaded up one of those to try. It worked just fine. My assumption was that there was an issue with the width of the magazines rather than the length, but I didn’t actually measure them. I guess I’ll have to compare the (relatively) high-buck mags with the GI-surplus and see how much they differ. In the meantime, the Norinco goes back into the bottom of the gun-safe since it can’t use the magazines I’ve standardized on for all the other .45’s.

    Thanks for posting this. It’s nice to know someone else has had the same kind of issue, even with completely different magazines (mine are all standard 7/8 round single-stacks).

  3. The problem with 2011 and their trigger bows, and to a lesser extent on the 1911 with the same problem, is each gun and each trigger would have a different go/no range.

    • I believe Joe shoots a double stack STI, with proprietary mags. My Para mags, for instance won’t fit an STI. Should be easy to machine a one-off mag check tester for an individual gun.

      • I think the issue is probably an out of spec (perhaps by damage) trigger bow. Tweaking the magazines by a few thousands were just an low risk way get around it. We’ll see when I get the trigger stirrup die from Brownells.

  4. Uncle was recently talking about the New Jersey cops suing Sig over the guns that NJ themselves ordered customized. Apparently the NJ cops didn’t test the custom design with their very different training ammo.

    And so once again we see a problem arise, or go un-diagnosed, from the practice of using one type of ammo for training and another type of ammo for duty. Or we could say that the issues arose from range rules that make using common ammo more difficult.

    So it is that I have come, slowly but surely, to despise organized shooting ranges. If the trend continues, shooting ranges will end up with all rules and no shooting. The best way for the range ownership to avoid lawsuits, complaints and regulations violations, after all, is to not have any guns (or eventually, not any people), on the property.

    Don’t get me wrong; I love structured shooting ranges, for other people. There should be way more of them than there are now. I just avoid using them myself.

    And I practice on my carry pistol using the exact same load as for daily carry. So as correct and true as the argument is for never carrying hand loads in your defense gun, there is one more argument in favor of it (as all the practical, as opposed to social, arguments are).

    Dagamore and Allen; there’s a thing called “stacking tolerances” that’s bitten us, and just about any manufacturer, more than once. For example, you have a threading tap that’s almost worn out, so it’s making slightly smaller diameter female threads. It’s in spec. Your fasteners supplier sends out a batch of screws that are on the large side of spec.

    Then it just so happens that you get a batch of parts from anodizing that have a slightly thicker-than-unusual coating. They’ve done that several times before and it was never a problem, but now, due to the other issues occurring at the same time, the screws suddenly don’t fit into the part. All the tolerances “stacked” in such a way that each stage in the manufacturing process contributed to the mis-fit. Hopefully you catch those things before product is shipped.

    In a final inspection of the gun in question, a pair of go, no-go magazines might have helped, but the test would have had to include a trigger function test with the large-ish “go” magazine in the gun.

    Also, I guess I’ll refrain from saying that, once again, a gun has failed due to the fact that it is such a high performance item, “because race gun”. This is probably not one of those instances.

  5. Joe,
    does your trigger have an overtravel screw? ( most aftermarket 1911 type triggers do)
    I have a vague recollection of a trigger problem that was indirectly of magazine origin. The mag catch slot was off in dimension, and this caused the mag catch to sit slightly offset in the frame. Where the overtravel screw tip would hit was uneven, and it would hit a high spot with this mag, and impede travel. But not every time, as some times the screw tip would slide past the high point.

  6. I had the opportunity to visit the STI factory (Georgetown, Texas) in 2005. At that time, they were making their own triggers and trigger bows.

    It occurs to me that if you brought this to the attention of the STI folks, they might be grateful for the ‘heads up’. Some of these components might have been made by a new employee, and it’s possible that a day’s run might not have been checked as carefully as they should have.

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