trajectory difference between bullet weight.

[Dos Cylindros]

Okay, here is my question;

Let's assume an AR-15 with a 50 yard BZO with 64 grain 5.56/223 ammo. How will point of aim, point of impact change with a 55 grain round? Will it hit higher or lower at 50? How about a heavier 77 grain round? I realize for combat shooting, there won't be enough difference to really matter, I am just curious.

[Molon]

When zeroed at 50 yards, the trajectories of the three loads you described will be virtually identical out to 70+ yards. However, when zeroed with one load and then firing another load without making adjustments to your sights, the points of impact may vary widely or hardly at all depending upon your particular barrel harmonics and recoil vector.

[Dos Cylindros]

When zeroed at 50 yards, the trajectories of the three loads you described will be virtually identical out to 70+ yards. However, when zeroed with one load and then firing another load without making adjustments to your sights, the points of impact may vary widely or hardly at all depending upon your particular barrel harmonics and recoil vector.

How widely, are you talking inches or less than one inch?

[AJS]

Inches.

If you look up OCW load development you will see just changing powder weight can have a large effect. By the time you start with projectiles and other factors you could be 6 or more inches out.

[Iraqgunz]

I remember back in '06-07' when I was in Iraq I zeroed my M16A4 hybrid w/ ACOG using 62gr. M855. Later I procured some Mk 262 MOD 1 ammo and was shooting to reconfirm and I wasn't even in the same ball park. I wanna say there was like a 2 feet difference. I thought my zero was screwed but when I loaded the M855 back up I was tits on. I ended up giving the ammo to someone else and sticking with the M855.

On a side note I would like to reinvestigate that scenario again.

[Failure2Stop]

Through my 18" Noveske, with a firm 100 meter zero with Prvi 75gr, 55gr Fiocchi and 55gr Wolf were both about 2" left, and the Wolf was about 1" high as well.
-shooting was 10 round groups at a rough 100 meters from support.

I am about to do some ammo comparison shoots, but in the past I have found that the most consistent aspect is inconsistency :p. You really need to confirm with your own gun, as different guns print differently, as does ammo manufacturer (and sometimes even lot).

[John Hearne]

I normally shoot 30/30 on our rifle qual. I attempted to qual with 64 gr power point after having carried a 55gr load and the elevation change in the rounds, in my gun, was enough to fail. I rezeroed with the 64 gr and shot it clean the second time.

[DocGKR]

One LE agency nearby has a large SWAT team whose AR15's are zeroed at 100 w/75 gr OTM, while the Patrol officers in this agency shoot 64 gr JSP out of their AR15's. Both groups conduct the majority of their training w/55 gr FMJ, as that is what is provided. Out to 100 yards, there is no problem hitting steel and 6" paper plates with any of the three loads...

[RogerinTPA]

I found that I needed to fine tune my zero when switching between 55, 62, and 75 grain ammo. I tested the 3 a couple of years ago and found a significant change in impact, when going from 55 gr Wolf BB, to 75 gr Wolf.

[TiroFijo]

One LE agency nearby has a large SWAT team whose AR15's are zeroed at 100 w/75 gr OTM, while the Patrol officers in this agency shoot 64 gr JSP out of their AR15's. Both groups conduct the majority of their training w/55 gr FMJ, as that is what is provided. Out to 100 yards, there is no problem hitting steel and 6" paper plates with any of the three loads...

Agrees with my experience, with several AR rifles with barrels from 11.5" to 20", there is very little POI difference up to 100-150 m when using 55 to 69 gr bullets.

[Dos Cylindros]

Thats what I figured, my issue round is 64 grain Win Ranger SP so that is what I zeroed my work and personal carbine for. I will try to just shoot this grain weight in my personal, with personal ammo, but good to know at normal engagement ranges it won't really make a big difference.

[Failure2Stop]

Just shot this today at 100 yards.

http://i296.photobucket.com/albums/mm174/Fail2Stop/e7d7fd62.jpg

Apologies for the lack of shots for the Prvi, I had just spent a while getting a new optic zeroed, so I guess you will just have to take my word that POI is good for it.

Top group is 06 manufactured M855 at the same distance with the same POA.

Groups were shot with an 18" Noveske from a bench with a less than optimal rest and a 3.5x TA11JG.

[shootist~]

Ammo variances from gun to gun can be quite significant or not so significant.

I was out last week shooting '09 IMI M193, which the 16" N4 is sighted for, plus some 75gr .223 TAP - both from the bench and using a sandbagged rest. The TAP was printing a good 3.25" lower at 100 Yds, which is a real pisser. Windage was close. An older lot of the M193 was ~1.5"R and 1.5" low verses the new.

[87GN]

The biggest difference I have seen - this is between Prvi 75gr at around 2650 vs handloaded 52gr Speer BTHP at around, IIRC, 3200 - was around 6" at 100 yards.

edit for 1"

[Failure2Stop]

Prvi 75gr at around 2650

Did you chrono this or is it a published velocity?
If you chronoed it, what barrel was it through?

[87GN]

Did you chrono this or is it a published velocity?
If you chronoed it, what barrel was it through?

ooooops. 2550. I chronoed it from an LMT 14.5". That was 08 production. The 07 stuff I had was significantly slower.

My handloaded 75s were 2650 and 2700. I was looking at the wrong slip. sorry.

[Dos Cylindros]

Just shot this today at 100 yards.

http://i296.photobucket.com/albums/mm174/Fail2Stop/e7d7fd62.jpg

Apologies for the lack of shots for the Prvi, I had just spent a while getting a new optic zeroed, so I guess you will just have to take my word that POI is good for it.

Top group is 06 manufactured M855 at the same distance with the same POA.

Groups were shot with an 18" Noveske from a bench with a less than optimal rest and a 3.5x TA11JG.

Nice pic and thanks a bunch. Correct me if I am wrong but M855 is 55 grain correct? Look to be a couple of inches difference at 100 yards between the heavier and lighter round. Again, obviously for "social" purposes it's nothing to get worried about.

[Failure2Stop]

Nice pic and thanks a bunch. Correct me if I am wrong but M855 is 55 grain correct? Look to be a couple of inches difference at 100 yards between the heavier and lighter round. Again, obviously for "social" purposes it's nothing to get worried about.

M855 is 62gr.


ooooops. 2550. I chronoed it from an LMT 14.5". That was 08 production. The 07 stuff I had was significantly slower.

My handloaded 75s were 2650 and 2700. I was looking at the wrong slip. sorry.

Ha, no problem. I am just trying to get good numbers since I am too lazy to do it myself :o

[BAC]

From a physics standpoint: if two outwardly identical bullets differing only in weight are moving the same speed and from the same angel, the heavier bullet will want to drop sooner. This is especially visible with a 200 yard zero, where you can see the rise and fall to the zeroed range, but even a basic ballistic calculator can confirm it.

The advantage of the heavier bullets is that they're also longer, and thus more efficient; they might start moving slower, but will retain their speed and resist wind drift better than lighter/shorter bullets moving faster initially. The lighter/shorter bullets will be flatter shooting at close ranges due to their initial speed, but drop off more quickly than heavier/longer bullets. The heavier/longer bullets start off slower, but will exhibit flatter shooting at extended ranges because they're more efficient moving through the air.

Not that any of this is helpful when determining exact differences in POI between M855 and Mk262. :D


-B

[TiroFijo]

From a physics standpoint: if two outwardly identical bullets differing only in weight are moving the same speed and from the same angel, the heavier bullet will want to drop sooner. This is especially visible with a 200 yard zero, where you can see the rise and fall to the zeroed range, but even a basic ballistic calculator can confirm it.


If two bullets have the same form factor (related to external shape), the heavier one has a better BC.

If the two are fired at the same muzzle velocity, the heavier one will not only have less wind deflection but also less drop (will shoot flatter).

But you are right in that all this is not helpul to determine POI variations with different loads.

You cannot simply take a ballistics program and compare the trajectories. The rifle is zeroed to one load at one distance, and the barrel harmonics also come into play. You have to shoot and compare the loads, and not all rifles (even seemingly identical ones) will show the same POI variation with different loads.