After tens of thousands of rounds sent down range in my 30+ years of shooting I had my first kaboom yesterday.  Actually I had 2.  Both were from the same package of rounds and both blew my mag out the bottom (the first shattered it).  The first one I thought was an accidental overcharge of the round but after the second I'm not so sure.  Looking at the spent cases I have several that are cracked and a few that are bulged.  The bulge is out the bottom of the case (where the firing pin hits) and not the side (like it would if the round were not fully seated).  I broke the gun down completely and cleaned it thoroughly.  I took an unused round in inserted it into the barrel.  It travels freely until the last 1/8" then it meets a little (and I mean little) resistance.  It can easily be pushed in all the way but  requires slightly more pressure.  I also recovered an unspent round from one of the mags after the kaboom that had the bullet pushed in the case.  I don't know if the gun caused that or if it happened during the kaboom.  The spec's on the gun are:

Ruger 10/22 modified to .17 Mach2
Volquartsen barrel and bolt
Kidd trigger

The gun has had approximately 100 rounds through it without a hiccup in this configuration until yesterday.  Any help as to what caused the kabooms would be greatly appreciated.

I keep seeing recalls on .17 with warnings not to shoot the ammo and the gun
 

That would be a HMR not a MachII

IIRC there is a reason you don't want a bottleneck cartridge in a blowback semi-auto, I've seen many posts explaining this. Apparently when they blowback they leave deposits that can prevent the bolt from closing all the way, causing an out of battery discharge. You probably need to clean the chamber thoroughly.

I know a couple good friends that hunt small game and target shoot ALOT with 10/22s that have been converted for 17HM2......it seems that an out of battery discharge is just common place and something that they have grown to expect now and then. Sounds like the Semi-auto with that round just isn't the ideal situation.

when I saw .17, I was like duh! Just word of mouth and internet stories, no personal experience but I'd say start there.

From the description, it appears that the bullet is slightly undersize and that is quite likely leading to bullet set-back as the cartridge is fed from the magazin and the bullet pushed into the case.

This can lead to substantially higher pressure and was always a concern for me when relaoding .357 SIG, I always selected bullets with a safe diameter and matched brass.

The issue you are having is the bolt is coming out of battery prematurely while chamber pressure is still high.  Hence, the blow out.

The heavy bolt handle and/or heavy bolt is supposed to keep the action closed until pressure levels drop.

I have Majestic Arms .17 HM2 installed with the stock bolt and Power Custom heavy bolt handle.  I haven't had a blowout yet, but I have seen some bulged cases, so my setup is borderline.  I think another 1/8 oz. of weight on the bolt or bolt handle would fix the problem.

My real issue are failure to extracts, probably due to the stock extractor, so I have Volquartsen extractor and spring kit waiting for me to install.

So far I can get dime sized groups at 50 yards and quarter sized ones at 100, as long as the wind isn't blowing.

Thanks to everyone for the replies.  I think I found the problem.  I checked the remaining rounds from the box I was using during the kaboom.  Out of approximately 25 rounds left, 5 had a split case.  After some research it appears this is common and my experience is also common.