400 fps loss?

[Red Wing]

Going back over my KD targets and notes again, and it's really bothering me that to get the calculator to match actual results, I have to reduce the fps from 2960 with a 24" barrel (straight from Hornady's site; 68 grain bthp match) to 2550 fps with my 16" barrel. I have the exact temperature and humidity from the day of the KD in the calculator. From what I've read, you're supposed to expect a ~25 fps loss per per inch, not 50 fps per.

So: is this within the realm of ‘normal', or are there other factors that could account for such a steep drop off?

Cheers!

[Burnett]

Without having tried to replicate your findings, I actually don't think it is out of the realm of possibility. A little more than I would expect, but reasonable.
Keep in mind too that to actually compare your results to Hornady's numbers, the bores would have to be in nearly the exact condition. Same type of rifling, same state of wear.
I guess if you can get consistent results, you are good to go.

[Ramblin' Wreck]

Part of the presentation at a KD event we run here in the Tennessee Valley is to caution shooters on relying too heavily on ballistic calculators. Even the best of them can only get you on paper. You still have to shoot each distance to verify where the holes are going to appear. Differences in environmental factors, platforms and different lots of the same ammo are going to give you different results. In a pre-shoot email We tell shooters to measure height over bore and do a calculation with the ammo (speeds) they'll be using and print out the ballistic table and bring it to the shoot. We then verify the data at each distance with sighters and WRITE IT DOWN. Some calculators like Strelok Pro give you the ability to go in and true up the calculation by entering actual data you got from your sighters. Your real data will be gathered from shooting instead of trusting the calculator. I rarely get real world results that match what the calculator says it should be but once I'm close I just make sure I have enough of that lot of ammo to complete the weekend.
I'm a big fan of Strelok Pro and have tables for every rifle I own but I always go verify before risking turning money into noise when a qualifying round is in my future.
Ramblin' Wreck

Going back over my KD targets and notes again, and it's really bothering me that to get the calculator to match actual results, I have to reduce the fps from 2960 with a 24" barrel (straight from Hornady's site; 68 grain bthp match) to 2550 fps with my 16" barrel. I have the exact temperature and humidity from the day of the KD in the calculator. From what I've read, you're supposed to expect a ~25 fps loss per per inch, not 50 fps per.

So: is this within the realm of ‘normal', or are there other factors that could account for such a steep drop off?

Cheers!

[scuzzy]

Part of the presentation at a KD event we run here in the Tennessee Valley is to caution shooters on relying too heavily on ballistic calculators. Even the best of them can only get you on paper.

Yup. And if you're getting your ballistics 'FPS' off the box of ammo just remember - they lie. Or maybe I should say they don't tell the whole truth of the matter.

Are you using a chrony to get your ammo FPS? Or what the box of ammo says?

At one KD I gotta box of Hornady 75 gr steel case. Wish they still made it - it was cheap and good. The box said it was  2900 FPS. The box of ammo didn't say what the barrel length was to get those stats of 2900. Well I was shooting an AR with a 16 inch barrel. My actual was 2480!! Under 2500!!!!. So every dang shot was low at 400 yards. Way way low. Heck I didn't know how low since it wasn't even on paper.

We then pulled out a chrony on Sunday (thanks Tim) and checked the velocity of the ammo. Under 2500 FPS. Yup. That'll screw you up.

And BTW - 55 gr works fine at distance.  Lot's cheaper too. Just do your calculations - BASED on the speed of your ammo. It'll work. But you NEED to know the actual velocity. Not what the box says. Yeah  - I know I know - you gotta use 62 or 68 or 75 grain for distance. Well not really. You just gotta know your rifle, ammo, and yourself.

And another BTW - go get some ammo here - really accurate - made by people that care about accuracy. And an instructor owned, and Texan, company who rock. Their 223 velocity is actually close to 556 - around 3180 or 3200 fps on the last batch. Cant remember but it was fairly hot. Accurate.

https://stand1armory.com/

Their 9mm is really good too. There may be an instructor discount. About to check.

Update: there is an instructor discount. PM me to get the code - it's 5% off and the price is already pretty good.

[Red Wing]

Thanks all for the insights and recommendations.

I definitely corrected based on actual results (and qualified on the first AQT after our DOPE run), just wanted to confirm that advertised and actual performance can be so completely different.

@scuzzy: I'll be looking to resupply in late summer. I'll PM you then.

Cheers

[LogCabinShooter]

From what I've read, you're supposed to expect a ~25 fps loss per per inch, not 50 fps per.

So: is this within the realm of ‘normal', or are there other factors that could account for such a steep drop off?

Cheers!

40 FPS per inch is a much better number to start with.

[Charles McKinley]

Put it over a chronograph. 
Your target is like your best friend, you may not like what it tells you but it will not LIE to you.  This is how this ammo performs out of YOUR rifle.  If you want better ballistics you can 1. Shoot a hotter round, 2.get a longer barrel 3. get a different upper in a caliber that is better at distance than 5.56.

As long as it is consistent learn the dope, write it down and fire for effect.

[Ramblin' Wreck]

Completely agree.  Once I find a load that works I stay with it till it quits working which usually means it's time to clean the rifle or get a new barrel. I test ammo using a Labradar and shoot on targets that give me the average velocity of a shot group at distances out to 1000 yards. It takes a lot of the guesswork out of ballistic calculations.

Wreck

Put it over a chronograph. 
Your target is like your best friend, you may not like what it tells you but it will not LIE to you.  This is how this ammo performs out of YOUR rifle.  If you want better ballistics you can 1. Shoot a hotter round, 2.get a longer barrel 3. get a different upper in a caliber that is better at distance than 5.56.

As long as it is consistent learn the dope, write it down and fire for effect.