Different speeds for different bullets of same weight?

[ztc92]

Hi All,

Fairly new to reloading and came across something interesting when truing the velocities for two different handloads out of my Tikka CTR 6.5CM.

I am working up two loads for it, the first with 130 TMK’s the second with 130 ELDM’s with hopes I can use the ELDM’s for range/practice and save the TMK’s for hunting.

Both loads are identical aside from the bullets: Starline SRP brass, 43.5g of H4350, CCI SR Magnum Primers, loaded to CTR mag length. Neither of these loads have shown any sign of pressure, though I found pressure with the ELDM’s at 44g of H4350 so backed off to 43.5.

What is interesting to me is that based on truing velocity at 1000 yards, the TMK’s are shooting about 50 FPS faster than the ELDM’s. I need an extra 0.2 mils of elevation at 1000 yards with the ELDM’s even though they are a higher BC bullet.

I’m guessing this is just a function of slightly higher pressures with the TMK? Seems unlikely the TMK would actually have a higher BC than the ELDM but I suppose that is a possibility too. Regardless, thought I’d see what you all think to make sure nothing is amiss.

 

[t_carlson]

Bullets have different bearing surfaces and jacket hardness.

Just chrony them at the muzzle and you'll know.

 

[Quicksilver]

What’s the jump to the lands between each bullet? Also what did they chrono at 100?

Eld bullets typically have a longer bearing surface which is going to grab at the rifling more causing slower velocities.

 

[ztc92]

Bullets have different bearing surfaces and jacket hardness.

Just chrony them at the muzzle and you'll know.

What’s the jump to the lands between each bullet? Also what did they chrono at 100?

Eld bullets typically have a longer bearing surface which is going to grab at the rifling more causing slower velocities.

Hadn't thought about increased bearing surface and friction causing the ELD to go a bit slower, that’s a good point. I don’t have access to a chronograph right now but I agree that would be the best way to confirm I’m not crazy. Thanks for your thoughts!

 

[wind gypsy]

Hadn't thought about increased bearing surface and friction causing the ELD to go a bit slower, that’s a good point. I don’t have access to a chronograph right now but I agree that would be the best way to confirm I’m not crazy. Thanks for your thoughts!

I'd guess its the opposite just based on form factor of ELDm and TMKs in other variants i'm more familiar with. I would have guessed TMK has a longer bearing surface and the ELD has less resistance to being squeezed down the bore and thus builds less pressure, and shoots slower. Could certainly be wrong.

What are you using for BCs to true them? AB curves?

 

[Quicksilver]

^^^ I have never held a tmk so wind Gypsy could be on to something as well. All I know is you rarely find a bullet that shoots the same. POI shift at 100yds I’ve seen substantial between different loads. Along with every barrel likes things different too.

 

[ztc92]

I'd guess its the opposite just based on form factor of ELDm and TMKs in other variants i'm more familiar with. I would have guessed TMK has a longer bearing surface and the ELD has less resistance to being squeezed down the bore and thus builds less pressure, and shoots slower. Could certainly be wrong.

What are you using for BCs to true them? AB curves?

This was more what I was thinking, one bullet (the TMK in this case) generated higher pressure and therefore more velocity.

I’m using the G1 BC’s provided by the manufactures’ websites:
- 130 TMK: 0.530 (a rough average of the BC’s provided for various velocities by Sierra)
- 130 ELDM: 0.5440 per Hornady

 

[wind gypsy]

This was more what I was thinking, one bullet (the TMK in this case) generated higher pressure and therefore more velocity.

I’m using the G1 BC’s provided by the manufactures’ websites:
- 130 TMK: 0.530 (a rough average of the BC’s provided for various velocities by Sierra)
- 130 ELDM: 0.5440 per Hornady

Looked quick at what AB has listed. They have the tmk listed with quite a bit higher BC
130 TMK: .273 g7, .533 g1
130 ELDm: .247 g7, .482 g1

That means my form factor guesses are probably wrong too.

 

[ztc92]

Looked quick at what AB has listed. They have the tmk listed with quite a bit higher BC
130 TMK: .273 g7, .533 g1
130 ELDm: .247 g7, .482 g1

That means my form factor guesses are probably wrong too.

That’s really interesting and probably explains what I’m seeing. When I use the G1 BC’s you provided to true velocity I end up with velocities that are much closer together.

Thanks for taking the time to look into this, seems that Sierra is spot on with their BC while Hornady is inflating theirs a bit…

 

[khuber84]

So my experience with a 130 eldm, trued g7 at 850y was 0.277, but this is from a 7.5tw 65cm, this waterlined at 660y and 400y. If you look through the pdm files on AB there is a rough average of 265-270g7 on the 130eldm samples. I have zero experience with a 130tmk. I know if I compare a 77tmk VS a 80 eldm in 22 cal, the tmk has quite a bit more bearing, closer to the 88eldm and 90 atip. Sierra usually has a harder jacket than Hornady and if the bearing is longer, I suspect wind gypsy's theory of reasoning on the slower eldm velocity would hold true. I do have some 95tmk 6mm bullets and the bearing is quite short, fair bit less than a 105 hybrid which is short bearing for weight. Pick the bullet that shoots the best and kill on.
To the OP, was wind deflection the same between the two bullets? A simple 0.2 mil elevation difference, could be a zero difference of say a 3 shot zero VS a larger sample size zero. There are so many variables to try and factor into this. A simple wind change from 2 to 6mph will cause 0.1 mil of aero jump at 1k yards.

 

[Crosser05]

Id be in the realm of bearing surface and jacket hardness.

 

[Formidilosus]

That’s really interesting and probably explains what I’m seeing. When I use the G1 BC’s you provided to true velocity I end up with velocities that are much closer together.

Thanks for taking the time to look into this, seems that Sierra is spot on with their BC while Hornady is inflating theirs a bit…

Hornady’s BC are from Doppler- they’re as close to any. AB chronically has lower BC for ELD-M’s than they are. Your issue is pressure- the TMK creates more pressure than the ELD-M’s.

I do have some 95tmk 6mm bullets

Want to sell some?

 

[khuber84]

Hornady’s BC are from Doppler- they’re as close to any. AB chronically has lower BC for ELD-M’s than they are. Your issue is pressure- the TMK creates more pressure than the ELD-M’s.

Some are close and some are not so close in my testing. If you use an average of their 3 given bc values of Mach speeds, it's a closer number to start with VS the box bc. Just my observations. Nah I'm not selling the 95tmk. The are a bit of an odd bullet, ideally they'd work well in a short neck case, or a short freebore chamber, something like a 243, or creedmoor with maybe 0.070 freebore. Touching lands in my 120fb 6gt, the bullet is 2/3 up the neck. I really wish they'd release a 105-110tmk. Be badass hunting bullet.