I have a situation occurring with a rifle that I can't explain and hopefully somebody can offer some help. It is a Remington model 7 SS in 7mm SAUM with a 2-7 Leupold Rifleman scope on it. On consecutive weekends I shot the 2 following loads. Same brass, primer, and powder was used for both sessions.

175 Hornady interlock, 53.0 grain of H4350 and velocity was 2692, 2698, and 2703. Zeroed this gun at 200 yards and 300 yard trajectory was minus 4 inches and 400 yard trajectory was minus 17 inches.

the next weekend using the same brass, primers, and powder I loaded 150 Barnes TTSX, 56.0 grains of H4350 and velocity was 2931 and 2943. I zeroed at 200 yards and 300 yard trajectory was minus 8" and 400 yard trajectory was minus 24 inches.

The velocity difference is about 200 fps and the BC is within .01 so why in the world is that slower bullet flatter? I zeroed both loads at 200 before shooting at 300 and 400 and temp was within 10 degrees both Saturdays. Has anybody else ever had a particular bullet/rifle/powder combo that was unusually flat like this? to get that same trajectory on the Hornady Ballistic calculator I have to up the BC to .55 and the velocity to 3050. Thanks for any ideas.

Can't explain why the 175 flies so flat, but I would guess (based on some experience) that the Barnes BC isn't nearly the .450 they suggest.

Be interesting to get the velocity at both muzzle and at the target.

BCs seem to be truly magical things.

The slower bullet isn't flatter. Whether slow or fast, heavy or light, bullets fall at the same velocity. It's a pretty common phenomenen for slower bullets to hit higher on a target and the reason is that the slower bullet gives the barrel just a tiny bit more time to rise before the bullet exits the barrel.

I do have a verified BC on the 175, but your results make no sense for either one.

If you have only shot a few rounds in load testing, and depending on how accurate your rifle is, you may not be able to accurately find the center of the groups with a small number of rounds fired.

There are too many variables to accurately predict trajectory without some additional testing. With limited test results you will only be in the ballpark.

Well most of the published tables are just mathematical predictions, the only real way of knowing its buy shooting and shooting a lot, put a lot more rounds down range, keep good notes on each shot fired along with, the Alt, Temp and humidity of were you are after a bit you will have a good average, make up your own chart and then just go hunting! I use to be into this sort of thing, but after a while, I figured that if I just do a 200 yard zero and keep on hair, I am going to make good hits as far as I have any business shooting big game! Have fun with it, after all its suppose to be a fun enjoyable hobby!

Not sure I follow this logic. They are both zeroed the same and shot at identical ranges. Pushing the same bullet to faster speeds with the same zero always results in a flatter trajectory with the faster load.

If you don't change the sights and at 'closer' ranges the slower bullet may shoot higher due to the phenomenon you describe. Happens all the time with pistols - if you don't adjust the sights.

It seems that more shots would tell the tale, but if they give the same result over and over, go with it! Nothing wrong with bullets flying flat. A lot of factors affect a bullet's bc in a given rifle, including altitude and temp, and the only real way to know what is going on is through repeatability.

I'd recommend you sight that gun in for 250 though. It will stretch your trajectory flatter farther. I get a mpbr of 300yd with loads going about 2700.

That is true if barrel movement from recoil is taken out of the equation.

Faster forward velocity means less time in flight and thus less time for the bullet to fall.

You may be overthinking this a bit....

This had me puzzled a bit....but looking at some ballistic tables, there is not a huge difference between the two....the OP's drop figures for the 150 and 175 seem to be reversed but are still within about 7 inches of one another at 400 yards.

I'd shoot some more.

Is the OP doing all this shooting on the same scope setting (power)?

This. Also make sure the barnes are 150 grains..

Hence, the continual comment from experienced people that "Your mileage may vary".

Every combination of components is unique to a given barrel and chamber. If I had to guess, I wouldn't, simply because I have a suspicion I can't prove.

To "Learn" why this is occurring, I would repeat the exact test as long as you have components and powder from the same lot. If any one component changes, the test is worthless.

If it repeats, do it again in a couple of months when the weather cools off and be sure to record the temperatures off your smartphone with the data.

John

It's my understanding that BC is a mathematical calculation based up on bullet shape and other factors. Optimistic calculations to support sales might be a possibility. I've never paid much attention to BC, giving more importance to other factors in my bullet selection. Mabe MD might add some thoughts in this regard.

Ed,
BC is also a calculation based on a particular velocity. Because the OP used the SAUM case, he is smack in the middle of commercially applied averages so the BC "should" be "representative" for "comparison" purposes within the caliber.

John

It's not a posibility with barnes, it's a fact. Still good bullets, though..

Good question.

If you're shooting zero out at distance, not using dots or dials, that wouldn't matter, right?

On some scopes it does.

If you take an older 3-9 Redfield and back the power off from 9 to 7 powder and you just raised you POI by 2 MOA. Most shooters never notice this because they only shoot on either the highest or lowest setting, which will typically have the same point of impact.

I haven't tested the particular scope mentioned in the OP to see if it suffers from this affliction or not, but it's a common one.

Ed,

While for many years most bullet companies calculated BC's, partly due to the very reason you suggested, many are switching to actually shooting them under controlled conditions. So it depends on the company.

On the other hand, recent research by Bryan Litz indicates the actual BC varies more to rifling twist than was previously believed. Actually, until fairly recently a lot of shooters felt too much twist "over-stabilized" a bullet, reducing BC, by Litz's research (involving actual shooting) indicates that over-stabilizing actually increases BC up to a certain point.

Litz's shooting essentially shows Hornady's listed BC for the 7mm 175 Spire Point Spire is spot-on, but I don't think he's tested the 150 TTSX. As noted earlier, however, I doubt its BC is the .450 Barnes lists. Among other things, Nosler's listed BC's are pretty good, and they claim .435 for the 150-grain Ballistic Tip, a noticeably sleeker bullet than the 150 TTSX. And Berger's BC (derived from Litz tests) for the .30 150 VLD is similar.

I don't know if this is what's in play here, but in the history of long range shooting, there is a phenomenon known as "compensation" where a barrel whipped differently for different loads, and released bullets at different initial trajectories. The Enfield with it's flexy rear locking action was one such rifle, and companies such as Fulton of Bisley (not Fulton Armory in the US) would "regulate" Enfields to allow rounds to converge at the desired range (generally 800-1000 yards). The standard barrel M14 was another rifle documented to "compensate" (though to a lesser degree than the Enfield).

My guess is if you play with BC's in JBM, you won't see the POI differences the OP is showing. I'd bet other than BC.

Tested BC is a reliable predictor of bullet performance, and inaccurate measurements and haphazard testing does not affect BC or how the bullet actually performs.