Perplexed, need some pro advice

[Chieftan]

Amateur with a lot of gun experience getting into more serious tuning of hunting rifles recently and learning a lot on the fly. Looking for a little help with a situation that doesn't make sense. Bought 2 rifles recently, .308 Win ("for my wife") and .300 WM, both Seekins Elements (love them by the way). Three thorough shooting sessions with both to ensure 100yd tuning (around 80-100 rounds through each of them). Mostly sub-MOA groups but I'm learning to be an above-average shooter.

So I move to the 300yd range to do a little drop test and reverse engineer my velocity as I do not have a chronograph. I ranged with a rangefinder at exactly 300 yards. The .300 WM shoots 5 shots (at scope zero) at an 8.2" drop average. I look at a predetermined Strelok Pro chart, 8.2" is 8MIL, take a shot at the bullseye and I'm almost dead center, 1" low at 300 yds. Barnes TTSX 180gr (I think 2960 velocity on the box)

Same bench, same bags used as a rest, same everything else, but the .308W shoots 5 shots at 7.3 drop average!! I'm immediately confused with less bullet drop with the .308 than the .300. From a lesser-tuned chart that suggests I dial up 16 clicks for 300 yards (MOA scope on this one), I dial up 16 clicks, take two shots that are 3-4" high.

I come home and reverse engineer the velocity using charts and the .300WM should be at 3010 equivalent velocity. Doesn't seem crazy at my altitude even with a short barrel (22"). HOWEVER, the .308 at 7.3" for 300 yards is at equivalent velocity of 3305!!

QUESTION: I'm at high altitude at 8200' so don't freak out over higher velocity with the short barrels but 7.3" of drop at 300 yards isn't even on any charts unless you're at 3300+ of velocity in a 150gr bullet. WHAT AM I MISSING HERE? Struck me that there could be something wrong in a good way with the barrel (MOA consistency so far at 100 and 300 is lights-out for a non-precision shooter) but maybe I'm just missing something big.

Conditions: 85 degrees, 20% humidity, 5-10mph wind at my back with 5-10 degree variations;

- Seekins .300 WM has Mark 5 HD, 22" barrel, Banish 30 suppressed; 1:10 twist; shooting off of bags. Bullets are 180gr Barnes TTSX BT
-Seekins .308 W has Zeiss Conquest V4, 21" barrel, Banish 30 suppressed; 1:11.25 twist; shooting off of bags; Bullets are 150gr Barnes TTSX BT
Shooting all factory ammo by the way

 

[akdad]

Did you only shoot them at those two distances?

 

[Chieftan]

Did you only shoot them at those two distances?

Yes, just now getting set up for longer distances and didn't want to move on until I have a better idea of velocity.

 

[akdad]

Try it at 150, 200,250 and you will have much better data. Just an idea

 

[Bucklowery]

For comparison

My 308 20" barrel @2550 avg. 168 g Bullets I am 0ed at 100
5.25 MOA@300. And. 9 MOA at 400 this is at sea level though

I would shoot some groups at increasing distances. You could have some bad information in your app

Thanks

Buck

 

[lancetkenyon]

Try this....beg or borrow a good chrono.
Go to a local range, and ask someone with a chrono if you can shoot a couple. If they are ANY kind of a good guy (or gal) they will be happy to let you shoot a couple. I have done it countless times (let someone shoot over my chrono) for guys that asked.

Also, 8.3" at 300 yards is NOT 8MIL. Maybe you forgot the "." for .8MIL.

 

[Ckleeves]

At 300 yards your gonna have a real tough time doing the "reverse math". With the 150 gr 50 Fps difference is worth .6" in point of impact at 300. So unless you shot a group with zero vertical then it's pretty impossible to get a accurate measure on velocity.

I used this method 15 years ago and it's a tough way of doing things. Like Lance said, beg or borrow a chronograph. If you can't then make sure your truly zeroed at 100, then shoot as far as possible (500 yards+) without dialing anything on a target tall enough to catch your impacts and then try the math again.

 

[Ckgworks]

I agree on the chronograph.....but here's some other food for thought when getting started shooting. You said that you where shooting mostly sub MOA, Which I'm reading into a little bit here so bear with me and don't think I'm fault finding at all.
I'm using inches for ease of example...... Let's say your groups are between 3/4" and 1 1/4" that's a 1/2" variable which translates to 1 1/2" variable at 300. If your spread is worse the variable goes up. All of these add up at some point and I think it's easy to get hung up on what apps and charts say we should be at when there's a ton of other variables. I think it was easier in the old days when we didn't have all the modern technology distraction. You shot, shot and shot more, built your dope card and went with it. I'd say keep shooting and use apps/charts to get you on paper. With shooting comes consistent confidence......I'm envious of the guys that can confidently shoot a Ladder work up at 500 yards without doubts of their shooting errors inducing bad data.
One other thing......I used to shoot alot of 100 yard groups and was very confident in my ability to shoot sub Moa, however when I begin shooting longer range the Sub MOA didn't immediately translate out. Most of it was me and I had to realize this and work on my shooting fundamentals instead of chasing my loads or rifle!

 

[JiminAZ]

Kind of a theory out in the weeds here, but do you have really tall scope mounts on the 308 compared to the 300WM?

Also recheck your zero at 100 with the 308. Sometimes scopes shift zero.

 

[cohunt]

How are you shooting? Free recoil- tight rear hold, downward pressure ?

Are you shooting both the same? Are both rifles the same weight? I'm thinking it could be the way you are compensating for the recoil on the bags for the different cartridges

 

[Chieftan]

Try this....beg or borrow a good chrono.
Go to a local range, and ask someone with a chrono if you can shoot a couple. If they are ANY kind of a good guy (or gal) they will be happy to let you shoot a couple. I have done it countless times (let someone shoot over my chrono) for guys that asked.

Also, 8.3" at 300 yards is NOT 8MIL. Maybe you forgot the "." for .8MIL.

Yes, thanks for the clarification for others.

 

[Chieftan]

I agree on the chronograph.....but here's some other food for thought when getting started shooting. You said that you where shooting mostly sub MOA, Which I'm reading into a little bit here so bear with me and don't think I'm fault finding at all.
I'm using inches for ease of example...... Let's say your groups are between 3/4" and 1 1/4" that's a 1/2" variable which translates to 1 1/2" variable at 300. If your spread is worse the variable goes up. All of these add up at some point and I think it's easy to get hung up on what apps and charts say we should be at when there's a ton of other variables. I think it was easier in the old days when we didn't have all the modern technology distraction. You shot, shot and shot more, built your dope card and went with it. I'd say keep shooting and use apps/charts to get you on paper. With shooting comes consistent confidence......I'm envious of the guys that can confidently shoot a Ladder work up at 500 yards without doubts of their shooting errors inducing bad data.
One other thing......I used to shoot alot of 100 yard groups and was very confident in my ability to shoot sub Moa, however when I begin shooting longer range the Sub MOA didn't immediately translate out. Most of it was me and I had to realize this and work on my shooting fundamentals instead of chasing my loads or rifle!

How are you shooting? Free recoil- tight rear hold, downward pressure ?

Are you shooting both the same? Are both rifles the same weight? I'm thinking it could be the

 

[Chieftan]

Thanks a TON for the questions and areas to consider everyone! Guess I'm out to make another purchase (chrono). Worse problems to have.
Some great advice on things to consider in shooting form.

 

[Philward]

Are the rifles zeroed at 100yds? Your post said you were tuning at 100 yds but didn't mention the zero.