New Phenomenon with Gen 4 G20

[GunBugBit]

I hadn't shot my G20 for over a year, but recently went out with a friend to try his new rifle build and figured I would bring my G20 along to try some Underwood loads I had never fired.

The Underwood loads were the 155gr 1500 fps ones.  The first few shots went OK and were on target.

On the fourth shot, the magazine dropped (or was forced) out of the gun.  I made absolutely sure the magazine was seated rock-solid and tried again, and it happened again.  My buddy was pretty sure I was inadvertently hitting the mag release button while shooting, but I carefully demonstrated to him and to myself that this was not the case.  It happened with 3 magazines, including when my friend tried it.  Of course, he was sure it was me until the same thing happened to him.  It appeared to us that the magazine was being forced out of the mag well.  What would do that?

The issue did not present itself when using the Hornady XTP 180gr 1180 fps load.  This load has always run 100% in this gun.

This gun never exhibited such an issue over the hundreds of rounds I shot out of it in early 2015, only a few misfeeds with hotter Underwood ammo.  But this was the first time I had tried this exact Underwood load.

The barrel is factory and so are all other parts.  Only the sights were replaced with a tritium set.

[The_Shadow]

Interesting to say the least, did you inspect the fired brass for damage or leakage?
The mag release catch or spring may have gotten damaged somehow or the magazine catch notch itself and it is unlatching and sliding free under heavy recoil.  Compare things to other magazines and the latch itself.

[DM1906]

Have a real close look at the ejected brass. Look for any case head separation, usually at the upper angle of the extraction groove, or at the top of the web. Anything that looks like a crack or split indicates escaping gas. The magazine well should also have some soot and/or powder residue, if that's the cause. It doesn't take much gas to blow out the Glock magazine, which is a good thing (it would split the frame and/or blow out through your hand, otherwise). Check this first, as it is a dangerous condition. If this appears to be a problem, it wouldn't be the first complaint of pressure issues with early UW 155's. More recent loads are reduced, significantly.

Definitely check the magazine catch, specifically for rounding-off or chipping at the upper side of the inner radius, opposite the button end. Check the mags for similar on the right/upper area of the catch-notch. If this is evident, reverse the button and try again. This will make it a "lefty", but will work for testing.

[GunBugBit]

Appreciate the fast, good responses!

The last week and a half have been busy with camping, traveling, other gun projects, etc., but I saved all the brass from that range session for later inspection.   I have had a look at the mag catch, mag catch spring, mag catch indents on the magazines.  The mag catch and spring look normal but one of the mags exhibits rounding/erosion at the top of the indent.

I have new spare mag catch parts to swap in and I have new magazines and a KKM barrel on order.

Will definitely have a look at that brass ASAP.

Thanks again gentlemen!

[GunBugBit]

My Underwood brass shows no signs of severe distress, such as case head separation.  However, it bulges at the rear more than the brass from the milder Hornady loads.  The bulges in the Underwood brass are visible with my reading glasses and the calipers show that the high spots of the bulges are bigger than 11mm, whereas the entire length of the Hornady brass stays below 11mm in diameter.  This did not surprise me.  Is it indicative of inadequate support in the Glock factory barrel?  The bulged areas would be inside the chamber when the round is in it, so maybe the chamber on that factory barrel is too loose near the mouth.

There is no more soot/residue in the mag well than I would expect.

Could the frame be battered so hard by these loads that it rattles the mag, mag catch and spring enough that the mag drops out?

It seems like I'm just a better-supported barrel and/or a stronger recoil spring (these are in the mail) away from avoiding this issue.  Maybe it's not even the barrel but I know I will feel better with a barrel that provides more complete support.

[4949shooter]

I have fired all the hottest loads from Underwood and Buffalo Bore through both of my Glock 20's (a Gen 3 and a Gen 4) without issue.

Your Gen 4 barrel should be fine as far as support goes. I would look for another cause to this magazine drop issue. Perhaps the mag is not being seated fully, or the mag catch spring is on the weak side.

[GunBugBit]

Thanks. For sure the mag was seated fully in all cases.  Am a competition shooter and know that I know how to make sure a mag is seated, plus my buddy cross-checked, plus I never had the issue until shooting the one particular load.

Mag catch spring could be weak I suppose but it was never a problem before.

I thank you kindly for sharing your thoughts to help me problem-solve.

[Pumpkinheaver]

Increased slide velocity with the new load along with a weak mag release spring could be your problem.

[GunBugBit]

I suspect a it's standard-strength mag release spring, just not up to the chore of keeping the mag release in its proper place when the recoil spring isn't strong enough to prevent battering.

I recently received some recoil springs to try, including a Wolff 24-lb recoil spring.  It's in the gun now, waiting for me to try it.

[GunBugBit]

Update on the Gen 4 G20

I was able to go out and try the gun with the KKM barrel and a Wolff 24-lb recoil spring, using 150gr (not 155gr) Underwood XTP loads stated to shoot at 1,500 fps.  I also shot about 70 rounds of Underwood 180gr XTPs shown on the box to move along at 1,350 fps.  So these are pretty hot loads.

I also shot about 30 rounds of Hornady 180gr XTPs, advertised at 1,180 fps.  These shot reliably in the gun from the beginning, and did so once again.  I'm happy these will cycle the gun with the 24-lb recoil spring.

The mag dropout issue appears to have gone away.  I should note that I now have an aluminum mag catch in the gun.  I don't like how extended it is because now the chance of operator-induced mag droppage is greater.  I can reshape the external button or go back to a factory part, and I'd even consider cutting down the factory mag catch button somewhat.  I personally do not need that big of a  button since my thumbs are not short.

There were a couple of failures to feed early on, which I attribute to the new barrel and a somewhat dirty and dry gun (my bad).  Things smoothed out as I got a few mags into the shooting.

A new issue that emerged is the slide locking back when the last round is at the top of the magazine.  This didn't occur when loading only 5 rounds in a mag, but only occasionally when loading 10 or more rounds in the mag.  My last string was with a full mag +1 in the chamber, with the Underwood 180gr XTPs.  All rounds fired except for the last one which sat on top of the mag with the slide inexplicably locked back.  A quick rack-bang was successful in every case where this happened.  I'll sort this out eventually I'm sure.

Now that I've cleaned the gun well, I'll report back after the next range session.

The Good
The trigger is really pretty good.  I have not done anything to it so it's supposed to be around 5.5 lbs, but it feels lighter and is not as mushy as other Glock triggers I've used.

The gun is comfortable to shoot with the hot loads.  I shot whole magazines one-handed, rapidly, and was not bothered at all.  I won't claim I can completely contain muzzle flip but still, the gun is quite controllable and causes no pain.  I use roughly the same grip pressure I use when shooting mild .45 ACP out of a 5" 1911, and it is enough.

I was shooting at a variety of targets and was hugely pleased how I can put bullets where I want them to go.  The 70+ competition matches I've shot have helped my overall handgunner skills, and it is satisfying to see them show up with the G20 shooting hot loads.

[4949shooter]

Glad you got it sorted out. The 24 pound spring helped my 20 as well with certain loads.

[RJM52]

I purchased a used G40 early this year that was only a  few months old. Using the Underwood 140 Xtreme Penetrator velocity was close to 1700 fps and the brass badly bulged. The ammo while hot looked fine when shot in some other 10mms including a G20.  In addition the trigger was not resetting properly.

Upon pulling the gun apart the previous owner had put in a Ghost 3.5 connector and parts.  I contacted the seller who "thought" he had included the original factory parts (gotta love home gunsmiths).  Once they were reinstalled all the problems went away. I then added a Lone Wolf stainless steel guiderod and increased power recoil sping and it even shoots better now.

Apparently the reduced power springs were allowing the slide to unlock sooner then the pressure dropped causing the bulges.

Bob

[GunBugBit]

Having had a chance to read more material from other G20 Gen 4 owners, I see that my issues are not unique to me and this gun.

I'd be pleased as punch if there were a simple remedy for the last round not feeding.  I've read about guys reshaping their slide stops, trying different slide stops, using stronger mag springs, etc., with varied success.

Clearly Glock did not set these guns up to handle anything beyond medium-hot 10mm, and they seem to want to stay out of the business of helping Glock owners validate problems they have with the hotter loads.

[Roguer]

OK don't know if this applies.

I tried field stripping that turned out to be almost impossible to get the slide off but some how it works sort ofishly.

But to get the slide on, I have to push the "Trigger bar?" all the way forward for the trigger to go forward before I can rack the slide all the way back to reset every thing. It won't go back all the way it stops even with the back of the gun.

No this gun hasn't even been fired, I just got it yesterday!

[Roguer]

OK after take apart where it seems to be a PIA slide removal that can be done after a few attempts each, seems some weird trick to get it done on this gun.

Then I got the slide to rack back several times to reset and lock up. But a few times I had to do the push the bar to push the trigger all the way forward to get the gun to rack back to reset. Trigger resets and the gun operates normally after that.  I have a feeling this isn't normal?

Edit: FOUND SOME THING!

OK...U-TUBE to the rescue!  Its the bump on the trigger bar!  Move slide to left when moving back then move back right.

https://www.youtube.com/watch?v=Mj8Qt8ofCAw

Still a PIA to field strip still...

Now that I find its a few years old problem and commonish it seems...mines a still a keeper!