measured myself with my cheapo Harbor Freight caliper, guaroonteed to be within .9 anytime its used.

Remember that is for a powder coated one. If yours is plain brass it will be thinner.

I just heard back from Palmer, and the official OD of the black powder coated barrel is 0.812 or 13/16.

Rogue42, you were pretty damn close! (the second time anyway )

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Tippmann A5 w/ Response Trigger - Apex Tip + UMS Apex Adapter + 12" Black Palmer Brass - 20oz CO2 on Remote w/ ProLine Pro Connect - Palmer A5 Stabilizer Kit - 8" Skeleton Stock - TechT Vortex and Lightening Bolt Mod - X7 Hopper - Madman Spring Kit - Lapco Offset Sight Mount - UMS Picatinny Grip - UMS A5 RIS MkII Front Tactical Mount

I have a super accurate micrometer somewhere... time to measure mine!

When I ordered my UMS APEX adapter I noted that I had an uncoated palmer brass barrel and added $5 custom work fee and FL^ sent me the adapter which works great.

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Anybody had their Palmers adapter/Apex pop off while shooting? Happened to me last outing.

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Palmer Brass/ Apex, Broadsword Vest, T4 Offset Hopper, Dogleg (original slab), A5A2 Foregrip, 114/4500 (Fits in my vest!!) NCStar 45mm Rad Dot < gonna make it work yet!
Glenn Palmer of Palmer-Pursuit.com
Best paintball field in Northern Virginia (or maybe all!) www.warplaypaintball.comCheck my review. PBReviews

So which one of these adapters does not come out. I've heard that the UMS one can be pulled off. Also I notices that the Palmers one comes with some sort of screws (to hold it on maybe???). Anyways at 15$ the UMS one looks like the one. Flightlines^ can you verify that your adapter cannot be pulled off before I purchase one please.

The only way to prevent either adapter from being pulled off is to machine the same grooves into the barrel that the Apex barrel has. EDit: Hmm, I seem to remember FL^ saying he could do the machining, as long as it wasn't a ceramic or fiber barrel.

The Apex/ adapter won't pull off easily, you have to want to. Why would you? Mine was paint covered. And I was too stubborn to stop and clean it. Up until that game it had worked just fine, like 6 cases.

And it's "FlatlinesUp!".

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Palmer Brass/ Apex, Broadsword Vest, T4 Offset Hopper, Dogleg (original slab), A5A2 Foregrip, 114/4500 (Fits in my vest!!) NCStar 45mm Rad Dot < gonna make it work yet!
Glenn Palmer of Palmer-Pursuit.com
Best paintball field in Northern Virginia (or maybe all!) www.warplaypaintball.comCheck my review. PBReviews

I've never had mine pop off while shooting, but I did get it snagged on a branch and it came off. Most likely it's becuase I'm using the Palmer adapter without the screws (they look goofy), so it's held on there by friction and a small piece of scotch tape

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Is it not sufficient to simply put the APEX over the barrel and screw it in place without an adaptor?

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It doesn't fit. The O.D. on the Apex barrel is 1". The Apex barrel also has a groove that provides a positive interface, there is a ring on the Apex that fits in the groove so it can't slide off. Most barrels have a 7/8" O.D. and the Palmers has 13/16" O.D. (I believe anyway). So there is a gap and it just slides around. There are several ways mount the Apex on these smaller barrels but they all vary in cost, quality, appearance and durability.