Awesome post!  this is the stuff I love to see, and what keeps me coming back.  I could have a beer with you sir!

Very interesting.  
When it comes to velocity, and especially velocity with M193 55 FMJ; it's fairly well studied that for a mil-spec cannulared 55 FMJ bullet, 2700 is about the magical fragmentation zone (or so they say, and make a good case).  So the way I look it, is >2700 FPS = highly effective, <2700 = much less effective.    And with that view, I actually look at effective range, vs velocity alone.  This really shows up in the more common 16" Barrels (or less), where spec M193 might be effective (i.e. >2700 fps) out to 150 yards, but weaker fair may only be effective out to 50 yards, before it's lower starting velocity puts it below 2700 fps.  So (and I haven't run the math), an extra 60 FPS might only seem like a tiny amount, but the difference between a 16" bullet starting at 3000 fps, and starting at 2940 fps, before the both fall to 2700 FPS, can be a fairly substantial percentage change in effective distance.  A 200 yard 16" AR15 is highly effective for almost all practical personal usage I'll ever have (that I would take a 16" to).  A 50 yard version is more of an indoor gun.

Or at least, that's one way to look at it.

Point being, even though 60 fps seems trivial out of a 3200 fps rifle length barrel; it can be more meaningful that it seems at first glance, by the above reasoning.

Thank you sir, if I am ever in your neck of the woods I'd be happy to grab a drink with you.

A detailed gel testing at distances was posted on this forum with Federal and Winchester M193 ammo.  Gel blocks were shot at actual distances out to 150 yards.  Each gel block also had a chrony in front of it to record actual velocity of impacting bullets.  They've found that 2,600 FPS was a more realistic fragmentation threshold for M193.  55 grain FMJ will actually fragment much of the time as low as 2,500 FPS, as other gel tests have shown.  Though it is not nearly as consistent, and the neck tends to get longer at that point.

http://www.ar15.com/ammo/project/Ballistic_Gel_Experiments/BTAmmoLabsTest1/Test1.html


Trust me, I am a velocity freak and I do view 60 FPS as significant for M193.  Though it is only in a few cases where the difference is that much between them.  For the South African M193, for instance, it was only a 25 FPS difference.  However, that could have been partly due to the barrel type too as the 20" barrel tested was a different make then the FN M249 CHF barrels.  I test a different 20" barrel next test and the results were....shocking!!  Just wait until you see!  Ideally, I'd want to test a 20" FN M249 CHF barrel like the other lengths were, but I didn't have one available.

What I can say for certain, regardless of the barrel type, the difference in velocity between 18" and 20" barrels is much less than the velocity difference between 16" and 18" barrels.  In fact, there is almost three times as much difference in velocity between 16" and 18" barrels as there is between 18" and 20" barrels.

I don't mean to give spoilers, but yes, it really is that significant.  And the 16" barrel tested is the exact same type as the 18" and 18.5" M249 CHF barrels.  Before testing, I was wondering just how much difference there was between a 16" and 18".  After testing, it reaffirms my desire for wanting at least a 18" barrel.  Though IMHO, 18.5" is ideal in terms of handling, performance, reliability, recoil, etc.  The unfortunate thing is that no one offers such a factory barrel, so it takes more expense and time to make one.

I was just thinking that it would remove variables that have to be adjusted for. In lots of the YouTube chrono tests I see, it looks like they would have plenty of room to position it at 26yds if they knew to

Well, like I said the point is to get muzzle velocity not 26 yard velocity.

Part II Has been Live, FYI

Thanks for this. I ordered a few more cans of PMP from Classic, because yolo.

Thank you for all the test.. I remember seeing this about a year back and carefully bought ammo that was in spec (before they years they went bad)    but, just use a 20 inch barrel.. i've never had an issue clearing rooms in close quarters using an A4

Great.  I've still got a couple k's of older XM193 (05-06) stock and a mix of 16 and 18.  IOW, it sounds like that's at least two different actual rounds.

That old brown box XM193 was wonderful.  Hot and more accurate than most M193 in just about everything IME.