Anyone with the Mayflower Gen V chest rig know if the larger outside radio/ IFAK pouches will hold a canteen or Nalgene?

I'm not sure that it would. The IFAK insert they have in the photos looks like the one that comes with the Gen VI Pusher and it looks snug in that pouch. The same insert is snug in the pouch on the Gen IV and that won't fit either of those bottles.

I can not get a 32oz nalgene in mine. I can get 3x AR mags though.  
The collapsible hydration pouches that are between 12-18oz fit in there though.

That's what I figured, thanks for the info!

Yeah I had the GenIV and the Pusher and that's one of the reasons they went down the road, I wish Mayflower would make the Gen V with a water bottle sized pocket.

You're not alone. Essentially the product needs to be a split-front chest rig with either an inherent, sewn-on water bottle/canteen GP on each side, or at least one. Your alternatives get limited once you want a split front.

I would look into the new Spiritus split from chest rig. If you added on a canteen pouch, like the VelSys jungle canteen GP, then other pouches as needed, you would achieve what you want. But speaking from experience with water carry on a chest rig, this is one of the fastest routes to a chest rig that is not only heavy, but kinda miserable to wear if you do anything other than walk. When that weight starts adding up it can be better to go to a belt kit with suspenders, or a more structural plate carrier system that better balances the load. All the chest weight pulls out on your spine and gets uncomfortable after some hours.

Just 1 liter of water is 2.2 lbs, whereas a full 30rnd mag of m193 is about 1.1 lbs. Keep that in mind when working off chest rigs that were typically designed for 3-4 magazines, an ifak, land nav items, and small accessories. Usually under 8lbs. The harnesses get to be the weak point.

You're absolutely right on the weight thing. I was looking at a chest rig as a lighter weight alternate, more so than my main rig for rural stuff. I agree the LBE concept is a better one for distance on foot and I have a British JAy Jays belt kit for that. I have a split front TT MAV setup I've used for years, I was mainly interested in the Mayflower because it's lower profile and lighter. I'll just stick to my Jay Jays for on foot and HPG heavy recon kit bag for driving. Thanks for your time and suggestions!

FWIW I love my Velocity/Mayflower stuff and have tons of it. I wish they’d modify their 14 hour assault packs to hook into their rigs like the HSP flat pack but like the rigs enough that I don’t mind wearing one and then a camelback if I need to.