I've been reloading for a little while now, but the one part I still struggle with is understanding & interpreting bullet behavior based on changes in bullet weight, shape, velocity, etc. On Sunday, a friend and I were doing some shooting at 200 yards. I was shooting a BCM 16" mid length with a 1:7 barrel and my friend was shooting a colt 6700 with a 20" barrel and 1:9 twist. We were trying out some loads he had made and were shooting at dinner plates stapled to the target boards. He indicated the reloads consisted of a 75gr HPBT bullet & 23gr of H4895.

 

We were both kind of surprised to find that the 75gr rounds were impacting notably lower than 55gr FMJ factory ammo at 200 yards. The difference was great enough that the point of aim that put the factory ammo about 1-3 o'clock on the plate would would miss entirely with the 75gr rounds impacting 2-4" low of the plate.

 

I was kind of surprised to see as significant a difference in POI at only 200 yards, but I'd be the first to admit I may simply not understand the subject very well. To those more experienced with bullet trajectory and such, does this sound right?

 

Thanks for any input.

I've been reloading for a little while now, but the one part I still struggle with is understanding & interpreting bullet behavior based on changes in bullet weight, shape, velocity, etc. On Sunday, a friend and I were doing some shooting at 200 yards. I was shooting a BCM 16" mid length with a 1:7 barrel and my friend was shooting a colt 6700 with a 20" barrel and 1:9 twist. We were trying out some loads he had made and were shooting at dinner plates stapled to the target boards. He indicated the reloads consisted of a 75gr HPBT bullet & 23gr of H4895.

 

We were both kind of surprised to find that the 75gr rounds were impacting notably lower than 55gr FMJ factory ammo at 200 yards. The difference was great enough that the point of aim that put the factory ammo about 1-3 o'clock on the plate would would miss entirely with the 75gr rounds impacting 2-4" low of the plate.

 

I was kind of surprised to see as significant a difference in POI at only 200 yards, but I'd be the first to admit I may simply not understand the subject very well. To those more experienced with bullet trajectory and such, does this sound right?

 

Thanks for any input.

 

 

WOW, I think you opened up a can of worms. As a rule as velocity drops your trajectory increases. I would think, and hope, the velocity on the 75gr BTHP was lower than the 55gr bullets. The thing I am really interested in was how much more the bullet dropped on your 16" barrel than your buddies 20" barrel.

I've been reloading for a little while now, but the one part I still struggle with is understanding & interpreting bullet behavior based on changes in bullet weight, shape, velocity, etc. On Sunday, a friend and I were doing some shooting at 200 yards. I was shooting a BCM 16" mid length with a 1:7 barrel and my friend was shooting a colt 6700 with a 20" barrel and 1:9 twist. We were trying out some loads he had made and were shooting at dinner plates stapled to the target boards. He indicated the reloads consisted of a 75gr HPBT bullet & 23gr of H4895.

 

We were both kind of surprised to find that the 75gr rounds were impacting notably lower than 55gr FMJ factory ammo at 200 yards. The difference was great enough that the point of aim that put the factory ammo about 1-3 o'clock on the plate would would miss entirely with the 75gr rounds impacting 2-4" low of the plate.

 

I was kind of surprised to see as significant a difference in POI at only 200 yards, but I'd be the first to admit I may simply not understand the subject very well. To those more experienced with bullet trajectory and such, does this sound right?

 

Thanks for any input.

 

How far can you throw a baseball (55g) and how far can you throw a bowling ball (75gr)? The constant is you(powder).

 

All other things equal, the heavier bullet will have a slower velocity and more drop at a given distance. The twist rate has little effect on the velocity unless it's not enough twist to stabilize the bullet. It's possible to have too much twist and cause the bullet to fly apart but that's difficult to do with normal powder loads.

The thing I am really interested in was how much more the bullet dropped on your 16" barrel than your buddies 20" barrel.

 

I can't say for sure if this was entirely due to barrel length, as he's got more of a long range optic and may have a more consistant hold, but there was a bit more of a drop with my shots. Collectively our shots were 2-4" low of the plate, but I think he was closer to 2" low of the plate and the lowest shots came from my rifle.

 

How far can you throw a baseball (55g) and how far can you throw a bowling ball (75gr)? The constant is you(powder).

 

All other things equal, the heavier bullet will have a slower velocity and more drop at a given distance. The twist rate has little effect on the velocity unless it's not enough twist to stabilize the bullet. It's possible to have too much twist and cause the bullet to fly apart but that's difficult to do with normal powder loads.

 

Ok. I suppose that makes sense. I think I had heard so much about how members of the armed forces are qualifying out to 500-600 yards with M4s that I had kind of assumed 200 yards was a trivial distance for the round and wouldn't really be enough time for a round to drop much.

 

I think my take away is that I can't half-arse it and get anything out of 75gr bullets. They'll only carry a benefit for shooting longer ranges if I take the time to get my rifle a proper 100 yard zero specifically for 75gr rounds and properly account for the different trajectory. I sort of realize now that I was [foolishly] assuming I could sort of get something for nothing by switching over to a 75gr bullet.

If the 55gr loads were also reloads and velocities of both were similar, you could *maybe* expect them to be a little closer. However, since you don't know the velocities of either and the powder characteristics of the factory ammo, its anyone's guess. His custom load could be rather mild and so that combined with the general rule that heavier bullets are typically loaded to travel slightly slower than thier lighter brethren in the same caliber, it understandable why they impact much lower.

If the 55gr loads were also reloads and velocities of both were similar, you could *maybe* expect them to be a little closer. However, since you don't know the velocities of either and the powder characteristics of the factory ammo, its anyone's guess. His custom load could be rather mild and so that combined with the general rule that heavier bullets are typically loaded to travel slightly slower than thier lighter brethren in the same caliber, it understandable why they impact much lower.

 

The factory ammo was Federal AE223. Federal website puts it at 3240 FPS muzzle velocity, but no clear indication of what length of barrel they're talking about. With the reloads, you're correct, I have no idea what kind of velocity we were seeing.

Also - what range are your AR15s sighted in to? 25/300? 50/200?

I would think that for the most part the slower bullet would drop more, whether or not it's heavier. Take away wind resistance and objects fall at the same rate. So in my opinion the bullet that's slower would drop further because it's in the air longer. There are a ton of variables though so you have to take them all into consideration.

Also - what range are your AR15s sighted in to? 25/300? 50/200?

 

Not 100% on his, but I had mine zeroed for 100yards and my [limited] understanding of standard .223 trajectory was that a 100 yard zero would be a smidge high at 200 and then close to zero again at 300 yards, which is part of why I was was that much more surprised to see the 75gr hit so low. But, again, I now realize that a zero based on 55gr factory FMJ isn't going to carry for a 75gr bullet with a different design & velocity.

Guys with little weiners will say size doesn't matter.

 

That being said....General rule under 24" you loose 50fps for every inch of barrel reduction. Over 24" you gain 25fps per inch increase.

 

Of course these are ball park and work for barrels between 16 and 30".

Not 100% on his, but I had mine zeroed for 100yards and my [limited] understanding of standard .223 trajectory was that a 100 yard zero would be a smidge high at 200 and then close to zero again at 300 yards, which is part of why I was was that much more surprised to see the 75gr hit so low. But, again, I now realize that a zero based on 55gr factory FMJ isn't going to carry for a 75gr bullet with a different design & velocity.

 

Sighting POA/POI at 25 yards will result in POA/POI at 300 yards. 50/200 is the same thing.

Sighting POA/POI at 25 yards will result in POA/POI at 300 yards. 50/200 is the same thing.

 

FWIW, a 25 yard zero shot at an actual 300 yard target in my experience has resulted in some pretty craptastic groups (to include complete misses). 36 yards will get you a 300 yard zero. Not much difference between 25 and 36 but that difference is magnified at 300 yards. Usually it's caused by some bad shooting habit that doesn't really manifest itself at such a short range.

 

Best bet is to just re-zero the weapon at 300 (using the 25yd BZO as a baseline).

 

But yeah in regards to the OP it's no surprise the 75gr round drops that much at that distance if you're basing it on a 55gr zero.

We were both kind of surprised to find that the 75gr rounds were impacting notably lower than 55gr FMJ factory ammo at 200 yards. The difference was great enough that the point of aim that put the factory ammo about 1-3 o'clock on the plate would would miss entirely with the 75gr rounds impacting 2-4" low of the plate.

 

 

Sighting POA/POI at 25 yards will result in POA/POI at 300 yards. 50/200 is the same thing.

 

 

FWIW, a 25 yard zero shot at an actual 300 yard target in my experience has resulted in some pretty craptastic groups (to include complete misses). 36 yards will get you a 300 yard zero. Not much difference between 25 and 36 but that difference is magnified at 300 yards. Usually it's caused by some bad shooting habit that doesn't really manifest itself at such a short range.

 

Best bet is to just re-zero the weapon at 300 (using the 25yd BZO as a baseline).

 

But yeah in regards to the OP it's no surprise the 75gr round drops that much at that distance if you're basing it on a 55gr zero.

 

 

OK, drop a 55gr bullet and drop a 75gr bullet. They drop at the same speed, just like everything else that doesn't have greatly differing friction from the atmosphere.

 

Now cram as much powder in a .223 case behind each that you can without blowing up the gun. You will notice the 75gr projectile moves a lot slower.

 

That means the time in flight to cover the same distance is shorter for the 55gr than for the 75gr projectile. That means the 75gr projectile has more time to fall before hitting the target. SO it impacts lower despite retaining more energy and being deflected less by wind (assuming everything else is the same, bullet profile can mess with both of those things).

 

As for the 25/300 and 50/200 zeroing thing and them not performing well, they are based on a given projectile (i.e. military 55gr projectiles), and a given velocity. Military ammo has a specified velocity range to not be rejected, and it's reasonably tight.

 

So for example, m193 ammo out of an m4 zeroed at 50 is about .3 inches high at 100, and about .7 inches low at 200. Zeroed at 25, it is about 5 inches high at 100, 10 inches high at 200, and isn't back at zero until around 350. m855 zeroed at 25 out of the same gun (heavier bullet moving slower) is at zero again at 300 yards. m855 out of 20" barreled m-16 zeroied at 25 is not back at zero until about 330.

 

For a 16" barreled gun, most factory 55gr is pretty well behaved with a 50 yard zero being within an inch and a half high or low from 25 to about 230.

 

If you want to know what to expect with your ammo, get a chrono and an external ballistics calculator. It makes life easier.

OK, drop a 55gr bullet and drop a 75gr bullet. They drop at the same speed, just like everything else that doesn't have greatly differing friction from the atmosphere.

 

Now cram as much powder in a .223 case behind each that you can without blowing up the gun. You will notice the 75gr projectile moves a lot slower.

 

That means the time in flight to cover the same distance is shorter for the 55gr than for the 75gr projectile. That means the 75gr projectile has more time to fall before hitting the target. SO it impacts lower despite retaining more energy and being deflected less by wind (assuming everything else is the same, bullet profile can mess with both of those things).

 

As for the 25/300 and 50/200 zeroing thing and them not performing well, they are based on a given projectile (i.e. military 55gr projectiles), and a given velocity. Military ammo has a specified velocity range to not be rejected, and it's reasonably tight.

 

So for example, m193 ammo out of an m4 zeroed at 50 is about .3 inches high at 100, and about .7 inches low at 200. Zeroed at 25, it is about 5 inches high at 100, 10 inches high at 200, and isn't back at zero until around 350. m855 zeroed at 25 out of the same gun (heavier bullet moving slower) is at zero again at 300 yards. m855 out of 20" barreled m-16 zeroied at 25 is not back at zero until about 330.

 

For a 16" barreled gun, most factory 55gr is pretty well behaved with a 50 yard zero being within an inch and a half high or low from 25 to about 230.

 

If you want to know what to expect with your ammo, get a chrono and an external ballistics calculator. It makes life easier.

 

Raz-o... Good job, great explanation. You have more patients than I do.

Yup - I zeroed using XM193 ammo - 55gr.