Difference between 69 grain SMK & TMK Reloading

[Nun_hunter]

Due to the shortage of Sierra TMK 69 grain in 223 I'm down to my last 100 rounds so have bought 500 69 grain HPBT SMK bullets instead.

 

For anyone who has done the same did you end up with the same powder charge for velocity and accuracy for the HPBT as you did for the TMK? 

From a bit of reading online it seems that going from a HPBT to an TMK may cause issues as the bullet is longer so seated further back in the case so may increase pressure but going the other way "should" be safer. 

 

I know I should start again and do it all properly but I'm interested to see what other have experienced and the sort of results I could expect.

[SMLE]

I’ve done the same thing, moving to the SMK at 69g.

I used the Hornady case length gauge thing to set seating depth and started at 30 thou off. Using my old TMK load of 22.9g of RS40 I found there was no meaningful difference in accuracy.

My Howa with this load is like a laser at 300 yards and holds about an moa at 600 if I try really hard.

I tried it with my other powder load of 24.8g of N140, and again, no real difference that I could tell.

No pressure signs or issues at all but please, do your research and work your load up safely.

[Laurie]

Using the bullet drawings in Bryan Litz's Ballistic Performance of Rifle Bullets 3rd edition, the two versions have similar bearing surface lengths and nose section radii. Both are true tangent ogive designs. On that basis, they would be expected to generate similar pressures and CBTO measurements at any given distance from the lands. The TMK does have an improved tail section, longer and with an improved angle to the boat-tail and this alongside the acetyl tip gives it a slightly higher BC.

By contrast the 77gn SMK and TMKs are identical bar the meplat/tip. Despite this Vihtavuori gives it another 0.8 grain N140 maximum in 223 Rem (and both exceed the maximum charge for very similar shape 77gn Lapua Scenar). I did wonder if this difference is 'real' or a result of testing at different times with two pressure barrels, but I did find on trying a few TMKs that they produce lower MVs than the old-fashioned (ie cheaper!) SMKs with same charges RS50 and same CBTO lengths. So maybe the 0.224 TMKs have thinner jackets generating lower pressures - they've certainly been picked up as frangible bullets by 'varminters'.    

[Nun_hunter]

Thanks both for very informative and detailed responses. 

I will get some more loaded once the bullets arrive and see what happens and if the results are as expected. 

It's interesting as a few places online suggest that the older SMK HPBT is more accurate (or at least easier to get better accuracy) than the newer TMK. Hopefully this is the case and I can reap the benefits of an accurate bullet that's cheaper than what I was paying. My rifle is a Howa Varmint barrel so fingers crossed I can get similar results to SMLE.

[SMLE]

“but I did find on trying a few TMKs that they produce lower MVs than the old-fashioned (ie cheaper!) SMKs with same charges RS50 and same CBTO lengths.“

Interesting Laurie. I found the same to be true using some crude chronograph measurements on my loads. With the RS40 powder loads mentioned above I was getting about 2770ish fps with the TMK’s and about 2830ish fps with the SMK’s. Not a particularly robust series of measurements but a difference noted none the less.