155 SMK velocity estimation

[Red_SC]

I'm switching from 175 SMK's to the new 155 SMK Palma bullets, and am running late getting everything together for a match this weekend. I ended up with 46.3gr H4895 in '72 Lake City Match brass, and CCI BR2 primers at about 2.820 oal. I finally got a chance to shoot it to get my drops at 525 and 1000, and from my comeups I got a velocity of around 2950 fps. The chronograph is saying it's running 2825, though, but it's an unknown Chrony so I don't know if it's correct or not. Currently my plan is to run the 2950 drop chart, and if I'm shooting under everything I'll switch to the 2825 chart.

Which velocity would you think is correct out of a 700P cut to 20"?

 

[MitchAlsup]

Re: 155 SMK velocity estimation

Which drag model was used to go from come-ups back to muzzle velocity? {G1, G7}

{Hint it should be G7 and use the Litz derived BC of the 155s}.

 

[ChadTRG42]

Re: 155 SMK velocity estimation

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Red_SC</div><div class="ubbcode-body">I finally got a chance to shoot it to get my drops at 525 and 1000, and from my comeups I got a velocity of around 2950 fps. </div></div>
This is your velocity. Actual come-ups won't lie (if the scope is tracking true). I'd run your come-ups with the given enviromental conditions, and plug it into your data for this weekend.

 

[BobinNC]

Re: 155 SMK velocity estimation

Red,

Hodgdon lists 46 grains of H4895 for 2873 FPS in a 24" pressure barrel. My gut tells me 2825 FPS smells right in a 20" tube; but I'd go with your actual comeup's. You sound like you know what your doing.

Good luck,

Bob

 

[deadly0311]

Re: 155 SMK velocity estimation

Red, when i was playing with the 155 scenars and H4895 i think i was running roughly 45.2 grains and was getting about 2750-2775 out of a 19 inch barrel, so your numbers sound about right.

 

[Red_SC]

Re: 155 SMK velocity estimation

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body">Which drag model was used to go from come-ups back to muzzle velocity? {G1, G7}

{Hint it should be G7 and use the Litz derived BC of the 155s}. </div></div>

I was using the G7 BC.

Thanks for the input, I think I'll start with the observed drop and go from there. The conditions should be pretty similar to when I was testing, so that will help some.

 

[Red_SC]

Re: 155 SMK velocity estimation

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: deadly0311</div><div class="ubbcode-body">Red, when i was playing with the 155 scenars and H4895 i think i was running roughly 45.2 grains and was getting about 2750-2775 out of a 19 inch barrel, so your numbers sound about right. </div></div>

Were you using thick or thin brass?

 

[Onemoretime]

Re: 155 SMK velocity estimation

I've got some Hbn coated ones laddered from 48-49 grains in .2 grain increments. 2.95 OAL (mag length for me)

QL says that 49 grains should be around 3000fps and 59107 psi. I've got a long Remmy chamber and that'll put me about .08 from the lands.

Keep us posted, I'm interested in how you do with them.

 

[Red_SC]

Re: 155 SMK velocity estimation

As an update, the lower velocity appears to be correct. This Leupold has quit tracking correctly, which could explain the odd observed drop. The first ten shots of the day were prone at 100. Shots 1-4 went perfect; shots 5-8 went 1 1/4 minutes low into a nice group, and then I started trying to get back on target.