I modified my AGC to make it ride higher and have a pistol holster on the back. I've made a few iterations of this setup for other pistols as well. I'm a leaner guy, so this setup keeps everything higher on my chest and the end of the barrel/holster between my pecs, only thing I had to change with my other gear was swapping an oversize zipper pull for a slimmer one.
On the AGC harness, swap all but one (the opposite side of the pistol) of the autolock buckles for Kifaru K clips, the older slim metal ones are best here. Trim the shoulder straps as short as you can at the top attachment points and restitch the binding tape to seal things back up. The harness now gets put on like body armor - Right arm thru the appropriate hole while sliding it on over your head, then grab the sole autolock buckle and attach under your left arm.
For the holster, you can use .080" Kydex and rivets/screws/nutserts (all available on Amazon). Go check out youtube videos for how to make a kydex holster, but about 30-40 minutes of work you should have something that resembles what you're after. Refine it a bit, then attach it to the harness with a sheet of kydex slipped into the back pocket and screws passing thru the holster & mesh to hit the nutsert on the inner sheet.
Retention on this holster is great, I've had it setup like this for two seasons and have had 0 issues with it. Sig P365 w/ TLR6 loaded with 147gr +P hardcast lead.
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