This last year I had my G20sf Gen3 slide milled by L&M Precision for an RMR. I also have a 6" kkm barrel installed but I'm not able to come up with a suitable holster for hunting. I'd like something along the lines of a UM84 or M12 flap style for protection but I don't believe they would work with the 6" barrel. Chest carry would be ok but would prefer waist. Any suggestions?
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Don't have much experience using a pistol for hunting purposes, other than a Ruger SBH in .44mag. For that I use a Diamond D Guides Choice Holster, it's made for chest carry. I know you stated this is less preferable than hip carry, but if you will be hiking long distances, it really distributes the weight well. This is doubly true if you are also carrying a rifle. Just not sure how the rmr will come into play. If it's not your cup of tea, I would go with OWB kydex.

I tried carrying my G21 a number of ways.  Finally settled on carrying my G27 in a Bravo Concealment RTT holster.  I don't notice it even when sitting in a deer stand but the pistol isn't for the game I have a rifle or cross bow for that.  It's strictly a PDW.

For woods/hunting/packing/survey use, I strongly prefer Simply Rugged and have been using Simply Rugged holsters for a good while now.  For EDC, I like Raven, but the SR is far more versatile and comfortable in the field for multiple days and over terrain, IMO. I have worked in the Alaska, North Idaho, Eastern Oregon/Washington backcountry for years and spend many days and nights outdoors with a handgun each year.  Always with a backpack with hip and sternum straps, and sometimes with a long gun, as well.  

A simply rugged Sourdough for revolvers or a Cuda (open) or Tribute (if you want a thumb break) for semi-autos is about as versatile as it gets.  Order with IWB straps (and Chesty Puller straps if you want chest  carry) and the same holster can ride OWB, IWB, cross-draw, and chest carry.  The chest carry can be adjusted to several cants and positions from midline to underarm, and 45 degree to straight.  In any position, SR are very quiet, which is great for hunting; and fast, which is great for defense.

FWIW, I recently got into G20s and spoke with Rob about retention for a Glock using a Cuda for chest carry.  With my other weapons, I prefer strong side, even with a 25-30L pack w/straps. But for mountain survey where I won't run into recreational hikers or for a heavy pack, fishing in chest waders, canoeing, sitting on my ass all night at the fire,... I like to have the chest carry option.  Rob used to carry a G20 in a Cuda on his chest while living/fishing in Alaska and recommends adding a tension screw, which he can do if you ask.  If you plan on spending most of your time in the chest rig, they recommend the Tribute.  

Either way, I imagine they will be able to accommodate your optic and barrel length (open bottom anyway) and retention preference with a phone call.

If you were going exclusively for chest carry, the Diamond D would be an excellent choice, though you're not and it does not have the versatility of the Simply Rugged holsters.

Simply Rugged also makes flap holsters if you really want one.  If you're looking for maximum protection and are still drawn to a flap holster, the Wilderness Safepacker would probably be a good solution for you as long as the Government size (they have +1" and +2") works for the overall length - I think you would be able to get the RMR in there without too much fuss, but don't know for sure. It's big-ish, but made for Search and Rescue backcountry carry, and will get you and your weapon to the same place at the same time.  For some weapons, the Safepacker may be a necessity, but the Glock is a Glock.  If you're carrying the G20 for defense against predatory people or critters, the Simply Rugged or similar holster will probably be faster into action and still concealable if need be.

I know I will deeply offend some here, but the M12 holster, in my personal experience, has not been awesome.  I used one as an infantry medic (read: running, humping, jumping, scrambling, crawling, etc.) for about ten years with an M9 before I permanently moved to a better solution.

Whether in training or downrange, I have drawn from the holster under stress, and it is not ideal.  I have also, on several occasions, had an M12 vomit my 'secured' pistol on its own while running, scrambling, or exiting vehicles.  I don't suck and I'm not new, and in my opinion, the holster (several units over the years) did indeed disappoint. If you go that route, I would suggest two things: a) don't; or b) use a lanyard.  Lanyards suck for backcountry and if in a thicket, will probably assist your weapon out of the holster (even a flapped M12), but at least keep you attached to both the pistol and the tree.  

I know Mainsail and others will disagree regarding the M12.  My experience is based on more than a decade of using multiple units of the same holster in an infantry setting, and is my own.  YMMV.

I can't disagree with actual experience, and while I've been hiking with mine (minor tumbles and falls) I haven't carried it in that extreme an environment.

It doesn't sound like the M12 will work with the RMR anyway, so for the OP this is moot.  

As for "running, scrambling, or exiting vehicles" those are all things I don't do with a backpack on- and it's the backpack belt to which the holster is mounted.  I did take a pretty violent end-over tumble about 50' down a steep hillside with my heavy overnight pack on, and the holster performed 100% - that may be due to the weak floppy pack belt having more 'give' than a web belt would.  


Not me.  I'm not offended, deeply or otherwise, by opinions and comments that are thoughtfully articulated.  It's the half-wits with no experience of a subject posting unsupported comments about a topic they don't really know much about that irritate me (and most people).  In this case you have experiential based comments, which I have as well (only with different experiences).  Which applies to the OP?  If I expected a 'running, scrambling, or exiting vehicles' type of scenario in my activity, I'd go with your comments hands-down.    

Mainsail, I agree and I'm glad I didn't offend.  The 'deeply' part was tongue-in-cheek, but I'm sure it does have its uber-fans.  Not pointing you out as an M12 cultist - just observing that your posts regarding their use with the G20 have been widely read and I wanted to offer an alternate view.  In fact, your thoughtful posts on THR contributed significantly to my decision to move to the G20 for backcountry use in the Northwest.