Ballistics puzzler - 7.62x39 handloads

[Newtosavage]

I've been tinkering with different bullets in my 7.62x39 loads over the past year. The best to date was the 160-grain Hornady FTX, both in groups and downrange energy.

Out of curiosity, I bought some 140 Monoflex bullets to try.

Accuracy was still very good - not significantly different than the 160's - with both loads shooting MOA at 100 and 200 yards.

So here's the puzzler...

Load 1 = 160 grn. FTX over 24.5 grains RL-7 @ 2250 fps
Load 2 = 140 grn. Monoflex over 25.5 grains RL-7 @ 2320 fps

Both loads zero'd to the same POI at 100 yards. BUT, at 200 yards, the 160's impacted 1.5" HIGHER than the 140's. That's what I can't quite figure out.

The 160's of course, have a higher BC which may be the answer, but according to the Hornady Ballistics Calculator, they should be about equal at 200. Why would the 160's land 1.5" higher? Anyone know?

 

[eastbank]

maybe because they are in the barrel longer(slower) while recoil is raising the rifle., but i would not worry about a 1.5 higher poi over the 140gr bullets at 200yrds.. eastbank.

 

[Newtosavage]

Oh, I'm not worried at all. Pleasantly surprised in fact, since that 160 FTX is delivering 1100 ft. lbs. @ 200 yards while the 140 Monoflex gives me just 960. I just expected the opposite to be true - that the 160's would be 1.5" below the 140's at 200.

 

[Casefull]

I'm with East Bank. It's a recoil thing. Hot loads in pistols will shoot low compared to the lighter loads.

 

[Newtosavage]

So why are they impacting at the same POI at 100?

 

[LoonWulf]

was velocity checked on the same day that the groups were shot?

 

[MCMXI]

So why are they impacting at the same POI at 100?

Possibly because your data set isn't big enough. What do you mean by the same POI. You need to shoot a number of groups (7 shots per group minimum), process the groups using OnTarget which calculates the group center, and then compare at least five of the 160gr groups to five of the 140gr groups before you can even begin to make a meaningful comparison. Also, you're adjusting your scope between groups so what's the resolution of your scope in terms of clicks? I notice that Hornady has two 140gr MonoFlex bullets, one with a BC of .277 and one with a BC of .335. From your comment re BC I assume you're shooting the .277 BC bullet. In theory, the heavier bullet should drop about 1/2" more at 200 yards with the same 100 yard zero.

I guess the short version is that your 100 yard zero for both isn't actually the same since barrel time aside, which is unlikely to be the issue in my opinion, if you have the exact same zero at 100 yards for both loads, the 160gr should be 1/2" lower at 200 yards and certainly not 2" higher ... assuming Hornady has the correct BCs published and your velocity data is sound.

 

[horsey300]

So why are they impacting at the same POI at 100?

100 yds for the 160 is still on the way up slightly and the combination of weight& bc is still carrying the path (as demonstrated by energy) longer, shoot em in between at 150 and see if the heavy weight isn't .75" high. Thinking of this cartridge in the terms of modern high pressure center fires is going to defeat you. Think of a .30-30 with 150vs 175 in trajectory. More of an arc than a line. Add in the aforementioned bullet in barrel time/recoil information, and you bet that 160 is gonna be higher longer. This is also another reason why ballistic calculator results should be thoroughly tested before use in the field, they just get you close. Sometimes the company lists the b.c. differently than real world results will yield as well. For instance, I find that nosler bc on the ablr is a BIT on the optimistic side of things, whilst Sierra sells their prohunters just a bit short. Test the velocity at the muzzle, then test it at 1-200 yds and put your info in an online bc calculator to see how close it really is, you may be surprised.

 

[MCMXI]

100 yds for the 160 is still on the way up slightly and the combination of weight& bc is still carrying the path (as demonstrated by energy) longer, shoot em in between at 150 and see if the heavy weight isn't .75" high.

What calculator did you use to determine this? Both bullets (assuming 140gr BC of .277) have virtually identical trajectories out to 200 yards according to ExBal and Applied Ballistics. I'm still thinking that this is a 100 yard zero issue where the OP doesn't have exactly the same zero at 100 yards.

 

[rbernie]

I'm also thinking that the advertised BC isn't accurate.

 

[LoonWulf]

Wouldn't it have to be way, WAY, off, to be that far different with velocity that similar?

 

[MistWolf]

Nevermind

 

[Newtosavage]

was velocity checked on the same day that the groups were shot?

No, but I've chrono'd those loads quite a bit, and they are always consistent. 2250 for the 160 and 2320 for the 140.

Could have been a tiny difference in POI at 100, but not enough for me to tell. I shot min. 5-shot groups with both, and wasn't adjusting the scope at all.

I also suspect the published BC figures aren't quite right but I have no way to determine that.

 

[macgrumpy]

Recoil has nothing to do with it- It's the bullet's weight.

The heavier bullet has more mass and therefore more momentum and retains energy/speed longer. This higher speed means that the bullet arrives at the target faster than the lighter bullet and gravity has less time to pull the bullet down.

Look at the predicted bullet velocity down range.
------------100yds-----------200yds
160 gr----2044fps---------1847fps
140 gr----2022fps---------1748fps

The end result is that since the 140 gr bullet is slower, gravity deflects it's trajectory downward more at 200 yards than the faster 160 gr bullet.

 

[horsey300]

What calculator did you use to determine this? Both bullets (assuming 140gr BC of .277) have virtually identical trajectories out to 200 yards according to ExBal and Applied Ballistics. I'm still thinking that this is a 100 yard zero issue where the OP doesn't have exactly the same zero at 100 yards.

I'm not using a calculator, I'm using past experience, and don't disagree with the zero issue either, I still recommend calculating the REAL b.c. first. Then chronograph loads same time of zero, then rezero and check every 50 yds for inconsistencies.

Wouldn't it have to be way, WAY, off, to be that far different with velocity that similar?

If the 140 is over estimated enough and the 160 is underestimated enough and then figure in 20gr worth of extra momentum, the bc difference could become an issue

No, but I've chrono'd those loads quite a bit, and they are always consistent. 2250 for the 160 and 2320 for the 140.

Could have been a tiny difference in POI at 100, but not enough for me to tell. I shot min. 5-shot groups with both, and wasn't adjusting the scope at all.

I also suspect the published BC figures aren't quite right but I have no way to determine that.

http://gundata.org/ballistic-coefficient-calculator/

Also for giggles try a 25 yd zero to compare different impacts at distance and see if your calculator is fooling you more than you realize.

 

[Newtosavage]

I still have quite a few rounds of each load ready to go, so I will shoot a few more at 25/100/200 and 300 to see if I can figure out what's up. If the 160 grainer is indeed going faster at 200, then I really don't have a good reason to use the lighter bullet at all.

 

[Another_Kelley]

What are the details on this rifle? - if you don't mind my asking...

A 7.62x39 bolt gun with a .308 bore sounds extremely appealing to me. - Is it in fact a .308 bore & what's the barrel length? - Thanks.

 

[Llama Bob]

That's what I can't quite figure out.

I agree - there is no reason that should be happening. I suspect there is some aspect of the problem that is mis-stated or hasn't been taken into account. Possibly related to the zeros not really being the same at 100y.

 

[horsey300]

I still have quite a few rounds of each load ready to go, so I will shoot a few more at 25/100/200 and 300 to see if I can figure out what's up. If the 160 grainer is indeed going faster at 200, then I really don't have a good reason to use the lighter bullet at all.

Something else just came to mind, and don't want to seem insulting but it's a plain question, for all shots fired, are you using the same magnification?

 

[macgrumpy]

I still have quite a few rounds of each load ready to go, so I will shoot a few more at 25/100/200 and 300 to see if I can figure out what's up. If the 160 grainer is indeed going faster at 200, then I really don't have a good reason to use the lighter bullet at all.

I don't know why anybody would question modern ballistics calculators, they may not be perfect but they are pretty darn close as long as you enter accurate information. It's important to research bullet performance through ballistic calculators and other resources before choosing. It's a common revelation for people that start to do long range shooting that heavier bullets quite often (not always but most often) perform better at long range. For so many years people were taught that light/fast bullets were better but it's not necessarily true. You have to compare BC values, velocities, and environmental conditions in order to find what will work best and in the end test results are always the final word.

 

[DownInTheDark]

Don't forget to take into account global warming. That can really affect a bullets trajectory. Ask Al Gore

 

[mshootnit]

Heck you can have different loads of the same bullet weight shoot that much different at 200. I am not sure weight can be said to be the root cause. You might try several powders with 160 and have that much variation.

Also in the velocity comparison above you are starting the 140 gr. slower and it finishes slower. What does that show?
The OP load in 140 is starting out 100fps faster

 

[Newtosavage]

What are the details on this rifle? - if you don't mind my asking...

A 7.62x39 bolt gun with a .308 bore sounds extremely appealing to me. - Is it in fact a .308 bore & what's the barrel length? - Thanks.

Sure, it's a Savage model 10 w/20" barrel. It was their original "scout" rifle and is drilled and tapped on the barrel for a steel rail and forward scope. I had it in that configuration at first, but eventually topped it with a conventional Leupold VX-1 2-7x33, which I think is the perfect match for the gun. And yes, it is a true .308 bore, but also shoots steel case .310 and .311 ammo extremely well. I get 1.5" groups all day with steel case Hornady 123 SST's (.310) for example.

 

[Newtosavage]

I agree - there is no reason that should be happening. I suspect there is some aspect of the problem that is mis-stated or hasn't been taken into account. Possibly related to the zeros not really being the same at 100y.

That could very well be the case. Looking at both 100 yard groups, shot on the same day, back to back, I can't tell any difference in zero. They are both the same elevation at 100 yards from what I can tell.

 

[Newtosavage]

Something else just came to mind, and don't want to seem insulting but it's a plain question, for all shots fired, are you using the same magnification?

Yes. Good question. All shots were taken at 7x on my Leupy