Hey mcseal2, I'll be curious to see what you come up with.  I now have the XDM 10MM in both 4.5 and 5.25.  I cannot get either one of those to jam in several trips to the range, not a single time.  Underwood 220 and 200 gr and my reloads all of which jam my G20 gen 4s consistently (both of them).  3 weeks ago I sold my 2nd gen 4 and picked up a G20 SF (which as far as I can tell is still mostly the Gen 3 version with just the gen 4 grip, no back straps).  I have only had that to the range once, but guess what, not a single jam, same ammo, Underwood (2 mags of 200gr) and my peppier reloads (6 mags) and stock mag springs, all mags fully loaded with 15 rounds.  Neither of the gen 4's would ever make it through those 8 mags (with me as the shooter...).  I didn't want to give up on the G20 just because there are so many options there for upgrades (barrels, sights, etc. both of which I already have), vs what's available for the XDM.  If my experience holds up on the SF, I will be swapping out the sights from my 1st Gen 4 and then selling it.  Luv the XDM's too, but the available options just are not there yet as far as I can find.  There are other threads on this sight where the Gen 4 was a problem.  Maybe the Gen 4's just take more experience to shoot (i'm going to choose to tell myself that's not it, and that there may be a flaw there) , but if the SF works with my level of skill, that's going to be my backpacking carry.

Sorry the double mag spring doesn't work for you.  I've been back to the range with it 2 more times now and have not had a single jam (I don't recall ever getting through a complete mag of Underwood without a jam before). I bought some of the wolff brand springs ($2 ea. if you buy 3 or more).  I got the 9 coil, that are what get used in the G 29 among others.  I cut one more coil off the bottom of the 9 coil reformed that bottom loop and added that in with the Stock G-20 mag spring and did not offset the springs, both were flush up against the follower.  Set two mags up that way and both functioned today with my peppy'r reloads, no problems.  Still could only get 13 rounds in the Mag. Also, the first time I tried this I had a 22lb recoil spring in.  The last two have been the stock recoil spring.  So for me, recoil spring does not appear to be the issue.  I hate having to rig this, but if it works...  you don't know how much time I have spent trying to get this to work.  Way to much.

On another note.  Since I am focused in on all of this right now, I went out yesterday and bought an SA XDM 10MM 5.25.  Took that with me today and shot one mag of the Underwood hardcast and 4 mags of my reloads.  No problems at all!!  I am going to give it a few more tries and if the experience holds I plan to pick up an XDM 4.5" as well.  I do admit that part of my particular problem could be my technique and perhaps the XDM is just more forgiving.

I was talking to a guy as we left the range today.  He had a new 9MM he was shooting today and I mentioned that I was there with the XDM for the first time. He said he had tried 10MM several years ago with a gen 3, G-20.  Said he finally sold it, because he could not deal with the constant jamming... he offered that comment up before I had ever mentioned my problems with the glock.

Just got back from the range again.  Offsetting the doubled up mag springs by 1 coil fixed my jamming on the second to last round problem.  I suspect there's a release of tension on the Mag walls on those last two rounds as they line up one on top of the other in the narrower neck at the top of the mag.  I figured offsetting the springs might reduce that at the longest extension of the spring.  Appears to work.

I rarely post, but I had a great experience today.  I've been trying to figure this same issue out as well for awhile.  G-20 Gen 4.  Finally bought another G-20 Gen 4 about 6 months ago to see if it was just the first gun.  Both guns perform just about identical.  My original is 3+ years old.  Have tried recoil springs in various weights, Wolff Mag springs, etc.  Nothing seemed to make a difference.  I know the common denominator is the shooter...  but I think I have good enough technique that, that is not the entire issue.  I have/shoot 3 diff M&P 45's as both my carry and home defense units and have no issues with them.  My accuracy is decent.  Was trying to get UW HC to shoot (220gr and 200gr).  After the new gun I decided to give up on UW and try working up my own load using Montana Bullet 200gr HC (have been following a couple of other threads on here discussing COAL, tipping etc. where others have had success).  But unfortunately that also has not been working. Typically have jams starting with the 1st to 5th round, usually multiple per mag for any of the more powerful loads.  Lighter loads, both my target reloads or factory have always worked fine.

Saw this thread this morning, Iceman's post in particular.  I have several extra stock mag springs left from the Wolff spring replacements I did so doubled up on the mag springs in two of my mags (I have 9 mags now, so it's not a specific mag issue) and headed to the range.  Got 13 rounds in the mag with a 14th in the chamber.  UW HC 220 and my latest reload attempt at 1.215 COAL 8.0gr Long Shot and every Mag I shot worked great until either the last or second to the last round.  Shot 6 mags and every one had a jam at the end, but performed without any problem until then.  For me this is a success story.  I have wanted this as my woods carry gun.  11 reliable rounds is OK by me (1 in the chamber, 10+ from the Mag with one or two left). Hopefully will never get to the jam except at the range if this continues to hold.  Interesting thing is that today on my reloads (shorter length than the UW) the jams were all nose up against the top back of the barrel,  never entered the chamber.  Normally the jams have been nose up, but part way into the chamber with the breach face a little over the round as Iceman described. I had a 22 lb recoil spring in, and am going to get a hold of a 19 or 20 lb spring. Will also lengthen the COAL on my reloads a bit and see if that helps in combo with the beefed up Mag spring.  Shooting the gun, there was no detectable difference to me with the extra spring strength when there is no FTF.

Thanks for sharing your experience Iceman, you helped me. I've been thinking of getting rid of the Glocks and trying a different make in 10MM, but I really like that I can swap in a 6 inch barrel if/when I want to (by the way, in my experience 220gr UW HC do not tumble with the 6inch barrel in either my glock or kkm barrel, a whole other topic).

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