It's been a couple months ago I shot three AR uppers on the same lower with the same ammo, optic, sand bag rest, 100 yard target distance and all just to compare.  First was an older Bushmaster Dissipater, thick chrome lined 16" barrel which is free floated, next was a new Palmetto State Armory 18" rifle length gas thin profile free floated and then a Bushmaster M4 profile 16" chrome lined non free floated which is the upper that shows a great variance in POI with different weight ammo.  The free floated uppers didn't vary POI that much.  I'd just like your opinions if you think the POI shift was more drastic due to the non free floated barrel of maybe that the M4 profile is a fairly thin barrel?  The 18" Palmetto State is fairly thin profile and didn't shift nearly as much.







Now before you guys call me out here, these pics are taken of a reprinted scanned target so there aren't actually any holes in the paper you're seeing.  I had scanned these targets in so they were PDF and I couldn't quickly and easily figure out how to convert to a format that Photo Bucket would recognize so I just printed the scans out and took pictures of them

Ammunition plays a huge role as does luck of the draw with rack grade barrels along with normal wear.  Look at the groups with your hand loaded 55 grain ammo, which produced very good groups in all three barrels.  To me an accurate barrel produces better accuracy generally regardless of ammo.  The Bushmaster chrome lined barrel produced good typical accuracy.  My opinion is your Bushmaster 16" M-4  wouldn't see a significant increase in accuracy if you switched to a free float hand guard.   Barrels seem to either have inheritent accuracy or they don't.  You pay a custom barrel maker for a cut rifled match barrel your chances of getting a very accurate barrel is high.  Your typical Bushmaster,  PSA, Spikes, are rack grade barrels IMHO and it boils down to whether you were lucky enough to buy a good accurate barrel.  All three of your barrels are minute of man and I'm not good enough to win a match regardless how good my equipment or barrels is.

I think you'll see a greater varition from ammo (bullets weights, powder charge, primers) whether factory or hand loads.  The despersion of the groups on the target in regards to where the groups impact on target is a result of many factors, recoil, barrel twist, harmonics, shooter influence and charge/bullet variability.   I'd be satisfied with any three of your barrels but I'm not super picky.  If you don't want any of them I'll provide my address.  

You didn't specify the barrel twist of your three barrels which is very important when shooting 75 grain ammo.  Some barrels twist is actually 1:9 some might be a little off on actual twist than what is specifically stamped on the barrel.  Some 1:9 twist barrels will shoot accurately with bullets heavier than 69 grains some won't.  If both Bushmaters are 1:9 twist this might explain the difference in accuracy with the 75 grain bullet.  If the dissapator is 1:7 and the M-4 is 1:9 twist I wouldn't be surprised by the results with the 75 grain ammo.

Results are not always black and white but some times there are underlying reasons like barrel twist or inconsistent shooting position or parallax variations with scopes.

Both of the Bushmaster barrels are 1:9 twist and the PSA is a 1:7.

I've been thinking of free floating the M4 but really would like to keep it light weight and probably just use if for any short range red dot type shooting.  If I thought that it would get rid of the drastic POI shift between bullet weights that it displayed compared to the other two then I'd certainly go that route.  I was pretty impressed with the 1 MOA group it did with my hand loads for not being free floated.  Those hand loads I worked up just for my Dissipater which nearly printed a 1/2 MOA there and has happened frequently when I get serious and am steady enough off sand bags.

I picked up a couple of PSA Magpul MOE lowers a month ago and would like to put together at least one upper for long range accuracy.  I'm thinking maybe an Odinworks DMR barrel or equivalent.  Any suggestions?  I would like to stay with a 16 or 18" length barrel and I'd say limited to a 600 yard plinker \ varminter \ predator gun.

It looks like you had great results with the 55 gr handloads all the way around, which rules out any differences in the guns - it points solely to the ammunition. So what are you asking? You didn't know that different ammunition shoots differently in every gun? You didn't know a marksman has to find the round that the gun likes and stick with it? Well, now you know. The 55 gr handloads shot great in every gun regardless of barrel type or hand guard type. If you want to shoot other kinds of ammo, great. All you have to do is re-zero every time you change ammo. When you find an ammo that shoots all over the target, you don't shoot it anymore - you try something else. Once you can shoot a tight group, you can then adjust your sights/optic to move the point of impact to the aiming point. Every gun is different, which is why marksmen will keep any gun that shoots. The ones that won't shoot with any ammo you get rid of.

I'm aware of most all you said here.  I was impressed that my hand loads did so well in each rifle but it's not the actual group sizes that I'm referring to or even concerned with.  What I'm looking at is the average POI from the 5 shots in each group, both my Dissipater and my PSA that were free floated didn't shift "average POI" much from the 55 grain loads to the 75 grain Hornady TAP rounds but the non free floated M4 upper shot about 2" higher with 62 grain bullets and 4" higher with 75 grain bullets vs the 55 grains and I was just asking for people's thoughts as to why this upper displayed so much shift in average POI than the other two.  It's the only one that's not free floated so that was my first thought.  If other's have experienced this POI shift then free floated theirs and had much less POI shift then I'd jump all over free floating this upper but if it's not likely to help much then I'll leave it as is.

If you have a chronograph run them over the screens.

The shift could be explained by velocity difference.  I am surprised only the non free floated shifted that much would expect the other two to be similar.

I wish I would have set up the chronograph that day but time was a bit limited that range session.  It sure eats up lots of time taking all the notes when load developing or comparing three different uppers with 5 different loads all with other variables being the same.

I'm just looking back at some of my logged data I keep from various range trips and have a few velocity measurements taken from my 16" Dissipator but they vary in weather conditions due to different days.

55 gr Hornady SP hand loads 3005 FPS weather 40°F / 30.13"HG / 51% humidity
55 gr American Eagle XM193 3180 FPS no weather data
62 gr American Eagle XM855 2949 FPS no weather data
62gr Green Tip PMC no data
75 gr Hornady TAP no data but Hornady website shows 2790 FPS out of a 24" barrel so my best guess is in the 2590 FPS range from a 16", maybe a little less considering Hornady likely wasn't testing with a semi auto action behind their test barrel.

Thinking external ballistics here, velocity will make a change but at only 100 yards I can't see anywhere near 4" of difference between these particular loads shot.  Now thinking internal ballistics, velocity is a larger part of the equation that will contribute to bore axis shift which is where my mind set is on this topic.  I'm pretty sure that the muzzle at time of projectile exit was different but wonder if more so due to barrel harmonics, rise from recoil or jump off of the sand bag?

My curiosity is about piqued enough to go ahead and free float the M4 profiled barrel and try this test again but second thought I've shot up all the 62 gr PMC and 75 gr Hornady TAP I had and don't plan to buy any more.  I'm certainly not insinuating anything wrong with either of that ammo, the TAP shot really well I thought.  I'm just focusing on my hand loads for accurate shooting and I have stock piled some XM193 before I knew it was the least accurate load.  I jumped on that XM193 for a decent price coming out of the ammo shortage a few years ago.  That shortage still isn't over for .22lr, still curious on that one.

You seem to know what you are doing when it comes to reloading.

What are the loads specifics?

FWIW, I thought your original post was very clear about what you were asking.  I think most people see pics of groups and then immediately go into telling you how you can do better mode.  LOL.  

I'm really curious as to what your results of FF your M4 barrel will be.  I've had a few skinny barrels, and more HBars.  All the skinny barrels I have had always shifted POI from load to load, more than my HBARS.  It's annoying.  For me, it makes it really hard to want to set up a SHTF gun with a skinny barrel, because if you're feeding it different kind of ammo, you want it to hit where you're aiming.   I have a Colt A2 20"er that throws POI so horribly from one load to the next, I only ever want to shoot 1 brand of ammo through it.  Which is not always feasible.  I used to stock up Q3131A WWB that was made in Israel and you can't get that any more.  And then 5.56 WWB now is always more expensive than XM193.  From what I can tell.  So my stash is kind of varied.  

I have a recce with an HBAR and an M4gery with an hbar and they have both shown to not throw POI from one load to the next.  Barely at all.  I like that.  But I don't like the weight.  I'd rather have a skinny barrel for my SHTF/ HD gun.  For HD, obviously it's not really an issue.  Inside the home you're not going to notice.  But I swear I've seen my Colt A2 throw POI 3" or more between brands at 25 meters.  Seriously.  At 100 yards that would be somewhere around 12".  Blaaaaaaa.  I had a Colt 16" carbine that was no different.  

So.....  those darn skinny barrels create the dilemma.  I've always been curious if free floating would help cut this down on a skinny barrel.  Or some kind of barrel harmonic dampening.  

The one thing that I'm not sure is a factor or not is the skinny barrels I've had, were always chrome lined.  And I wonder if that doesn't play a factor here.  I did have an SAK barreled A1 upper for a short bit and it seemed to be tamed a little in this regard.  It shot really well too.

ETA:  The one thing you got going for you is that most of the POI shift seems like elevation.  And they're doing what I think is typical.  Higher POI for a heavier load.  My Colt will do elevation and windage. Grrrrrrrr....  

And FWIW, you get the right FF tube, they sometimes can be lighter than the standard Handguards.  So....  there's that, since you were concerned with adding weight.  Some are heavier though.  You just have to check.

I just got lucky with my hand load there as I didn't do much load development for this cartridge.  Currently using WSR primers with TAA once fired surplus 5.56 brass, Hornady 55gr SP flat base and a near max load of H4895.

I don't think you'll achieve top velocities with the H4895 but I found that it worked good in my .308 and .22-250 so I stocked up on it a year ago when I finally run across some 8# jugs of it.  I just got fortunate that it works equally as well in my ARs.  I've not had any cycling issues or primer problems but I've really not run an AR hard either so I can't say how they would work at a high fire rate.

I've just recently caught the AR bug and looking forward to putting together at least one accurate AR for longer range plinking steel and to hunt some coyotes.  I'm actually really impressed with the accuracy of my Bushmaster Dissipator upper that I free floated.  From all I've read a Chrome lined 5.56 chambered 16" carbine gas shouldn't shoot like it does but it does print well when I can hold her steady.

JJREA, Thanks for sharing your experiences with the thin and heavy barrels.  I think I will free float it, probably won't be real soon as I have so many other projects going on right now but I'll get back on it and do my home work on a light weight rail.  The little I've been studying on them I really like Midwest Industries M-Lok rails for their looks anyway, haven't compared weights of any yet.

You shot 3 barrels, and got different results, unless you FF each then test, you cannot tell what caused the differences, anything else is just guessing.

He is talking about the shift in POI on one specific barrel.   The other two only factor in because they show they don't shift as much as the one in question.

It would be a good thread if you showed if there are any differences once you free float it.  Although it seems like this particular subject goes right over peoples heads.  I'm not sure why that is.  Maybe people just shoot one kind of ammo all the time.  I don't know.  I've always bought boxes here and there to try out for accuracy and never really bought a lot in bulk.  So I get to see the shift.  Either that or everyone else's barrels don't shift between brands and we're the only ones.  

yes-you cant use those for comparison=not same barrel/setup