175 SMK’s with .170 Freebore

[Florida Wally]

Has anyone tried with any success shooting 175 SMK’s with a .170 freebore .308?

 

[6bra1k]

are you mag length restricted ?
the throat is at the neck when seated at 2.82

 

[Florida Wally]

Single shot, so not mag restricted. Setting up rifle to shoot 200-20x bergers. I also have a bunch of 175 SMK’s and wonder if they will shoot.

 

[Nowhere Man]

Probably not very well. 200's like a 1-10 twist while 175's prefer a slower tw.

 

[Laurie]

Single shot, so not mag restricted. Setting up rifle to shoot 200-20x bergers. I also have a bunch of 175 SMK’s and wonder if they will shoot.

They almost certainly will shoot well, although you may need to do a sort and batch job with a comparator on the BTO measurement. (Recent SMKs are excellent in this respect; older ones not so good.) The blunt 7-calibre radius nose on this design and R/tR value of 1.00 (perfect tangent form) make this a very, very tolerant bullet as to chamber form and jump-size.

The problem is that the short low-radius nose creates a LOT of drag compared to a good modern long-range bullet, so it loses speed rapidly down-range and is more wind-affected. Comparing the 200.20X and 175 SMK gives G7 BCs of 0.328 v 0.243. As they're different weights which affects final BC value, a better metric is the 'form-factor' which compares how much in-flight drag they generate compared to the G7 'reference projectile'. The 200.20X tests out at 0.919; the 175 SMK 1.085. the reference projectile is always given a value of 1.000, so the Berger produces c. 8% less drag in flight; the Sierra produces getting on for 1% more.

A lot depends on the distances you intend to shoot over. The 175 Sierra will shoot to 1,000 and beyond, but is getting pretty slow usually. To stay above 1.2 MACH (c. 1,350 fps) which is roughly the upper figure for the transsonic speed zone, you need an MV of around 3,025 fps under standard ballistic conditions (59F 29.92 inches mercury pressure). Each 1 mph crosswind change moves the bullet nearly 9 inches so getting for 1-MOA per mph wind change. The 200.20X at 2,650 fps MV is travelling around 100 fps faster at 1,000 (1,450 fps) and moves a shade over 7-inches per 1 mph crosswind change.

Figures from Bryan Litz's Ballistic Performance of Rifle Bullets 3rd ed., and Berger online ballistics calculator function.

 

[Florida Wally]

They almost certainly will shoot well, although you may need to do a sort and batch job with a comparator on the BTO measurement. (Recent SMKs are excellent in this respect; older ones not so good.) The blunt 7-calibre radius nose on this design and R/tR value of 1.00 (perfect tangent form) make this a very, very tolerant bullet as to chamber form and jump-size.

The problem is that the short low-radius nose creates a LOT of drag compared to a good modern long-range bullet, so it loses speed rapidly down-range and is more wind-affected. Comparing the 200.20X and 175 SMK gives G7 BCs of 0.328 v 0.243. As they're different weights which affects final BC value, a better metric is the 'form-factor' which compares how much in-flight drag they generate compared to the G7 'reference projectile'. The 200.20X tests out at 0.919; the 175 SMK 1.085. the reference projectile is always given a value of 1.000, so the Berger produces c. 8% less drag in flight; the Sierra produces getting on for 1% more.

A lot depends on the distances you intend to shoot over. The 175 Sierra will shoot to 1,000 and beyond, but is getting pretty slow usually. To stay above 1.2 MACH (c. 1,350 fps) which is roughly the upper figure for the transsonic speed zone, you need an MV of around 3,025 fps under standard ballistic conditions (59F 29.92 inches mercury pressure). Each 1 mph crosswind change moves the bullet nearly 9 inches so getting for 1-MOA per mph wind change. The 200.20X at 2,650 fps MV is travelling around 100 fps faster at 1,000 (1,450 fps) and moves a shade over 7-inches per 1 mph crosswind change.

Figures from Bryan Litz's Ballistic Performance of Rifle Bullets 3rd ed., and Berger online ballistics calculator function.

I have noticed the difference in BTO in the older SMK’s. I once had a box of 500 with a 0.012 ES. Drove me crazy trying to get a constant CBTO when seating. I have also noticed that lately the BTO ES is 0.002 to 0.003. Glad to hear they are jump tolerant.
Appreciate the response Laurie!

I forgot to mention, I’ll only be using them in midrange club matches, so mostly out to 300 yds and a few 600 yds matches.

 

[Ned Ludd]

The main issue you may run into is that you may not be able to seat the 175s all the way to "touching" and still have much bullet shank left in the case neck in a chamber with .170" fb designed for the 200.20X bullet. The difference in BTO dimension for those two bullets is ~.060". If this turns out to be the case, simply determine the maximum COAL/CBTO measurement that leaves ~ half a caliber of bullet shank seated in the neck (or whatever amount you deem "sufficient") and use that as your "zero" or "reference" seating depth. You can then test seating depth unidirectionally, moving the bullet incrementally only in the direction farther away from the lands. As Laurie noted, the 175 SMK tangent ogive should be relatively forgiving with regard to seating depth, so you will hopefully be able to find a seating depth that not only shoots small, but also leaves sufficient bullet shank in the neck.

 

[Florida Wally]

Thanks Ned. The seating method makes perfect sense. Similar to seating based on mag length.