Look into the Mec-Tech (I believe it is). They have a Carbine upper that uses your pistol lower and mags. I haven't fired one but it is interesting especially if you have, say, 2 Glocks. You can use one as is and use the other lower on the Carbine or you can pick up a Lone Wolf Dist. frame and have it dedicated to the Carbine. KOOL with 33 round mags.
FWIW
Richard
I got a good price on the Mech-Tech a couple of years ago from a third party distributor. It came with the standard skeletal metal frame stock with a butt pad and I ordered the M4 type collapsible stock.
I had trouble removing the old stock because it takes a VERY thin wall 3/8 box or nut driver. I shelved that project due to other priorities and have just recently started up on it again. I used a Dremel to cut the old stock frame away and was able to remove it with the tools I had. However, the same type box wrench was needed to install the M4 type stock.
I contacted Mech-Tech and spoke to Frank Jones, General Manager, who called me in response to my email.
They have an adapter that you can use with a threaded M4 stock. They are exchanging the one I purchased 2 years ago for this new one. They just received my return yesterday, so we will see how soon they get the replacement to me.
Mr. Jones said that they use a Skill 3/8 box for this removal and install and it works. Most modern tools are cheaply made and cannot be that thin with the poor material they use.
He offered the exchange because he didn't like me to have to purchase a Skill tool just for this.
I purchased a Glock 21SF and used my plain Glock 21 frame for the Mech-Tech.
Not sure how their product will perform, but I'm pleased with the customer service so far.