longbeard Posted March 18, 2017 Share Posted March 18, 2017 Hello all, I have a sporadic issue with the last bullet in my STI 140mm magazine failing to feed by going nose up on the last round in the mag. Very recent grams 11 coil mag springs, grams followers are older. Feed lips appear in spec (.385 - .390), 14# recoil spring. Newly fit firing pin stop, and extractor tension is reasonable. Maybe one in 500 rounds does this. It's happened with two different magazines. I cannot consistently reproduce the issue. Managed to snag a picture today, the case head is completely out of the magazine which means the extractor didn't catch it somehow. This is an STI edge < 10K rounds on it. Ideas anyone? Link to comment Share on other sites More sharing options...
Steve in Allentown PA Posted March 21, 2017 Share Posted March 21, 2017 On 3/18/2017 at 3:18 PM, longbeard said: I have a sporadic issue with the last bullet in my STI 140mm magazine failing to feed by going nose up on the last round in the mag. This is a bolt-over-base malfunction caused by inertia feeding. What's happening is the last round in the magazine is not firmly up against the underside of the feed lips when the slide comes forward. Instead of being engaged at the rim, the slide is contacting the case ahead of the rim and pushing it forward until it is released from the mag. At that point the base of the round goes down, the nose goes up, and the now vertical cartridge is pinned against the barrel by the slide. Bascially, the slide is coming foward faster than the magazine spring can push the next round up into position to be properly engaged by the slide. Inertia feeds become more common as the number of rounds in the magazines decreases. This is because the magazine spring exerts less force as the number of rounds in the magazine decreases. Common corrective action includes replacing the magazine spring with a stronger or new one, insuring there is no oil on the follower or underside of the feed lips, fitting a flat bottom firing pin stop to slow the rearward velocity of the slide, installing a stock 23lb mainspring or a stronger than stock one to reduce the rearward velocity of the slide, installing a shock buff (5" pistols only) to reduce the impact force of the slide against the frame, and/or installing a lighter recoil spring to slow the forward velocity of the slide thus giving the magazine spring more time to push the next round up firmly against the feed lips. Exactly which of the above is the right one is unknowable. I would start with the simplest, cheapest one first and work my way down the list until the problem is fixed. Personally, I'd do the magazine spring first. That has a very good chance of being the cure. At the same time I'd perform the simple maintenance task of making the inside of the mag bone dry. Link to comment Share on other sites More sharing options...
B585 Posted March 21, 2017 Share Posted March 21, 2017 I doubt I can be of much help, but I have experienced this with factory ammo. If I use my reloads, it doesn't happen. In my case, I am almost certain the problem is due to the short OAL of factory ammo. Any chance you are using a short OAL? Even with factory ammo, that doesn't happen when I use my new MBX mags. The SVI mags I have trouble with have very old springs which is what I suspect problem is. Link to comment Share on other sites More sharing options...
longbeard Posted March 21, 2017 Author Share Posted March 21, 2017 1 hour ago, Steve in Allentown PA said: This is a bolt-over-base malfunction caused by inertia feeding. What's happening is the last round in the magazine is not firmly up against the underside of the feed lips when the slide comes forward. Instead of being engaged at the rim, the slide is contacting the case ahead of the rim and pushing it forward until it is released from the mag. At that point the base of the round goes down, the nose goes up, and the now vertical cartridge is pinned against the barrel by the slide. Bascially, the slide is coming foward faster than the magazine spring can push the next round up into position to be properly engaged by the slide. Inertia feeds become more common as the number of rounds in the magazines decreases. This is because the magazine spring exerts less force as the number of rounds in the magazine decreases. Common corrective action includes replacing the magazine spring with a stronger or new one, insuring there is no oil on the follower or underside of the feed lips, fitting a flat bottom firing pin stop to slow the rearward velocity of the slide, installing a stock 23lb mainspring or a stronger than stock one to reduce the rearward velocity of the slide, installing a shock buff (5" pistols only) to reduce the impact force of the slide against the frame, and/or installing a lighter recoil spring to slow the forward velocity of the slide thus giving the magazine spring more time to push the next round up firmly against the feed lips. Exactly which of the above is the right one is unknowable. I would start with the simplest, cheapest one first and work my way down the list until the problem is fixed. Personally, I'd do the magazine spring first. That has a very good chance of being the cure. At the same time I'd perform the simple maintenance task of making the inside of the mag bone dry. Great answer. Much Appreciated!! I had come across inertia feed info since posting this and came to the same conclusion, I'm glad someone confirms it. The mag springs are 4 months old, and just not anywhere close to EOL. I'm going to tune the feed lips a bit as they are a slightly wide, and and make sure no oil is on the mag. After that it's springs if it happens again. Id go springs first, but I used the last batch for a solid year. 4 months just seems such a quick life. Link to comment Share on other sites More sharing options...
CHA-LEE Posted March 21, 2017 Share Posted March 21, 2017 How old are the followers? Followers don't last forever and worn out ones can dramatically reduce the registration speed of the ammo as it's stripped out of the magazine. I would suggest trying all new followers and mag springs regardless of how "used" you think they are. I have seen brand new mag Springs start to fail with very little use. Some times you just get a bad spring. Link to comment Share on other sites More sharing options...
longbeard Posted March 21, 2017 Author Share Posted March 21, 2017 9 hours ago, CHA-LEE said: How old are the followers? Followers don't last forever and worn out ones can dramatically reduce the registration speed of the ammo as it's stripped out of the magazine. I would suggest trying all new followers and mag springs regardless of how "used" you think they are. I have seen brand new mag Springs start to fail with very little use. Some times you just get a bad spring. Thanks, I had wondered about the followers as well. They have about 15K rounds on them. If the problem persists I am going to replace all mag guts. Link to comment Share on other sites More sharing options...
jcc7x7 Posted March 21, 2017 Share Posted March 21, 2017 Looking at it overall, mag guts are cheap. I've put new grams in and have had them cause feed issues in a previously working mag. then installed another set of the same parts from the same order and had them work fine. So I don't rely on just the fact that there new! always test your mags springs and followers. IMO Link to comment Share on other sites More sharing options...
Gman57 Posted March 25, 2017 Share Posted March 25, 2017 On 3/20/2017 at 8:41 PM, Steve in Allentown PA said: This is a bolt-over-base malfunction caused by inertia feeding. What's happening is the last round in the magazine is not firmly up against the underside of the feed lips when the slide comes forward. Instead of being engaged at the rim, the slide is contacting the case ahead of the rim and pushing it forward until it is released from the mag. At that point the base of the round goes down, the nose goes up, and the now vertical cartridge is pinned against the barrel by the slide. Bascially, the slide is coming foward faster than the magazine spring can push the next round up into position to be properly engaged by the slide. Inertia feeds become more common as the number of rounds in the magazines decreases. This is because the magazine spring exerts less force as the number of rounds in the magazine decreases. Common corrective action includes replacing the magazine spring with a stronger or new one, insuring there is no oil on the follower or underside of the feed lips, fitting a flat bottom firing pin stop to slow the rearward velocity of the slide, installing a stock 23lb mainspring or a stronger than stock one to reduce the rearward velocity of the slide, installing a shock buff (5" pistols only) to reduce the impact force of the slide against the frame, and/or installing a lighter recoil spring to slow the forward velocity of the slide thus giving the magazine spring more time to push the next round up firmly against the feed lips. I've experienced the same issue and agree with your diagnosis! Link to comment Share on other sites More sharing options...
Steve in Allentown PA Posted March 25, 2017 Share Posted March 25, 2017 5 hours ago, Gman57 said: I've experienced the same issue and agree with your diagnosis! So how did you solve the problem? Link to comment Share on other sites More sharing options...
Gman57 Posted March 28, 2017 Share Posted March 28, 2017 It only happened once and on the last round of one mag. Never did anything about it other than ID the mag. I need to check feed lips and spring tension. G Link to comment Share on other sites More sharing options...
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