PCC: IRL Trajectory and Ballistic Calculator Do Not Match . . . Why?

[twofewscrews]

This all day long.

I wish. Play with the numbers. My velocity would have to be off by 500fps and even if my sight height was off by half an inch, up or down, it still wouldn't explain it.
The calculator is right using 25, 50, and 100 yard zero's. I've used "proper" scopes before and had such a wild deviation, but other then getting on paper I also never shot closer then 25 yards.

 

[Walkalong]

I wish

If the numbers don't match results, it's bad data in. Just figure out what you put down wrong.

 

[bfoosh006]

Your velocity might be low.

FWIW... here is a ton of OEM 9x19 chrono data from my 16" Colt Pattern PCC.
https://www.ar15.com/forums/ar-15/-/16-712312/?page=1&anc=7429025#i7429025

 

[ExtremeSquared]

...

The only time I can establish a "true" zero with this rifle is within 10 yards. Past 10 yards the group opens up no matter what position, benched or supported, bagged or bipod, to at least 1 to 2 inches. For example, the first shot will be zero, second shot high right, third shot low but centered, fourth shot high right, fifth shot low but centered. Because of this I shoot 10 shot groups, between 10 and 25 yards, and so long as the spread stays within a 2 inch group I call it zero. From 25 to 50 yards a 3 inch spread is normal. At 100 yards the spead was roughly 4 to 6 inches. Its a takedown model so I assume this shot to shot variation is a product of the barrel not actually being secured to the reciever. All my other rifles do not suffer from this issue and zero is actually zero.
...

I had this until I tightened the barrel takedown to "tighter than expected". Beyond "firm", just approaching the edge of difficult.

 

[Dr T]

With deference to LoonWolf's initial observation, it could be that the model is a bit off. When it is model vs observation, observation always wins.

Also see http://www.columbia.edu/~to166/hiawatha.html

 

[Varminterror]

Individul ballistic reality seldom match the numbers

This really, REALLY is just not true...

This science is OLD, we've been calculating time of flight based on drag coefficients to determine trajectories for a long, LONG time... All of these calculators really derive their results from the same basic math, same few formulas, and it's REALLY rare that they're actually WRONG, with the singular exception being:

If the expected outcome doesn't match what you're seeing in real life then your data input isn't accurate.

 

[Varminterror]

Is this a function of it being a takedown? ie because its a takedown the barrel is flopping about throwing rounds high . . . or something of this nature.

Were it this, then you would not be able to repeat your zero at 17yrds. Whatever mechanical dispersion occurring at 50yrds would be happening still at 17yrds, not preferentially favoring high impacts at one distance and then returning to zero at another.

I'm much more apt to bet that at 50yrds, that 2.5moa thick line of the PA Prism (2.5moa thick LINE, not 2.5moa tall chevron) is floating high a bit more than you're trying to hold. We might be compounding it with an optic height error, as every .1" of optic height in a 17yrd zero will push a 50yrd strike by 1/4", and we might be stacking a little more velocity error (50fps errors push another .2"), but if we're 2" off, using a macro aiming point on a macro target, I'm betting MUCH more on sight picture error than ballistics.

But I do agree with the other folks. A 17yrd zero is about as appealing to me as drinking dogbowl water...

 

[twofewscrews]

I had this until I tightened the barrel takedown to "tighter than expected". Beyond "firm", just approaching the edge of difficult.

I've played with the takedown tightness level before. I'll give it another shot though.

 

[twofewscrews]

If the numbers don't match results, it's bad data in. Just figure out what you put down wrong.

I've never actually verified that the 50'6", 25 yard, and 50 yard targets are actually where they say they are. I assume the 25 yard and 50 yard target are correct or close enough as the trajectory from the ballistic calculator basically matches. Next time I go to the range I'll have to measure the distance from the bench to the targets, and then from target to target.

 

[twofewscrews]

Were it this, then you would not be able to repeat your zero at 17yrds. Whatever mechanical dispersion occurring at 50yrds would be happening still at 17yrds, not preferentially favoring high impacts at one distance and then returning to zero at another.

That's basically what keeps me wondering, cause if its not a mechanical error, then its either data or me. I'm sure on all my data but target distance. Only one I have no verified.

I'm much more apt to bet that at 50yrds, that 2.5moa thick line of the PA Prism (2.5moa thick LINE, not 2.5moa tall chevron) is floating high a bit more than you're trying to hold. We might be compounding it with an optic height error, as every .1" of optic height in a 17yrd zero will push a 50yrd strike by 1/4", and we might be stacking a little more velocity error (50fps errors push another .2"), but if we're 2" off, using a macro aiming point on a macro target, I'm betting MUCH more on sight picture error than ballistics.

For sure a possibility. As I mentioned in another reply to someone, I did not have this same kind of issue when using a scope.

But I do agree with the other folks. A 17yrd zero is about as appealing to me as drinking dogbowl water...

I use(d) a 17 yards zero because, according to the ballistic calculator I use, a 17 yard zero puts the bullet .6" high at 25yards , 1.4" high at 50 yards, .08" high at 75 yards, and 3.4" low at 100 yards.
I very much liked the idea of not having to think about holds and still be able to place the bullet in to 6" plate out to 100 yards without thinking about it.

 

[Walkalong]

I've never actually verified that the 50'6", 25 yard, and 50 yard targets are actually where they say they are. I assume the 25 yard and 50 yard target are correct or close enough

Mt buddy and I got burned on the 300 yard range here getting .22s set up. I had to come out one afternoon and mark it from 50 to 300 (At least 300 was right) in 25 yard increments.

 

[Varminterror]

I use(d) a 17 yards zero because, according to the ballistic calculator I use, a 17 yard zero puts the bullet .6" high at 25yards , 1.4" high at 50 yards, .08" high at 75 yards, and 3.4" low at 100 yards.
I very much liked the idea of not having to think about holds and still be able to place the bullet in to 6" plate out to 100 yards without thinking about it.

That seems a little off for that velocity - what are your atmospheric inputs?

 

[Varminterror]

Here in KS today, 17yrd zero would be a 100yrd zero for a .145G1 at 1345 with a 2.0” optic height.