Range Report BC/Performance Question: 155 Sc. Vs. 175 SMK?

[earthquake]

Was going to post this in another thread here after reading this quote,

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Pusher591</div><div class="ubbcode-body">Thanks for the replies but after shooting in some windy conditions and I'm here to tell anyone that doesn't beleive it...heavy is the name of the game if you want to buck the wind!</div></div>

but figured I'd start my own since it got a little long.

I have a couple questions about this. I currently shoot 175 SMK's out of my 20" Bartlein and I'm getting 2,550 fps average. I keep hearing (and seeing at the range and comps) that more and more people are going with the .243, .260 or 6.5CM at high velocity (2800+) I think, saying they "ride" the wind better. Seems they're of the speed over weight crowd.

In perusing B. Litz's ballistics book that a friend owns yesterday, I was comparing the 155 Scenar to the 175 SMK. I'd read that the Scenar is a better (faster?) round for .308, but unless I'm missing something, I think I read that the 175 SMK had a higher (<span style="font-style: italic">better?</span>) G7 BC than the Scenar.

I also read in the book how faster/lighter bullets perform better at short ranges, but the heavier/slower ones catch up performance wise at extended ranges because they retain velocity better....

I was thinking of picking up some 155 Scenars to experiment with in getting more velocity, but now I'm not so sure. What am I confusing here?

 

[mkollman74]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I am looking into this as well. I haven't fully processed some new info I found, but it is intriguing. Litz wrote an article where he addresses the BC vs speed issue. He states that for every 0.1 in lower BC, you need 496 fps to make up for it. Not yet sure if that applies to G1, G7, or both. Anyhow, it's a good and practical ratio to keep in mind.

 

[BryanLitz]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mkollman74</div><div class="ubbcode-body">I am looking into this as well. I haven't fully processed some new info I found, but it is intriguing. Litz wrote an article where he addresses the BC vs speed issue. He states that for every 0.1 in lower BC, you need 496 fps to make up for it. Not yet sure if that applies to G1, G7, or both. Anyhow, it's a good and practical ratio to keep in mind. </div></div>

Someone please tell me what article the above statement is from.

Thank you,
-Bryan

 

[Rthur]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

COfox, you have the attention of "the" guy concerning ballistics. Now would be the time to speak up.


cheers Mr Litz

 

[Pusher591]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

OP, the statement you quoted was from my thread regarding 155 VLD/Scenar loads. I'm some what new to ballistics, but not shooting so in my mind I thought the a faster bullet would mean the wind has less time to act on the bullet.

This is true in a way but I was trying to make somehting happen that never will. I will never be able to safely/accurately push a 155 Scenar type bullet @ 2900-3000 fps from a 17.5" barrel

Now if I could, I still don't know if it would help "as much" because the 175SMK & 155 Scenar (I use those bullets becuase proably the most commonly used) are in the same BC arena, not exactly matching but very close.

Now, when it comes to the 243/260/6.5 crowd they start off with significantly better BC's and are able to push those particular bullets faster, bascially a win,win. I think thats why they "ride" or "buck" the wind better then the heavier .308 cartridge.

My comment was made strictly in reference to the .308 cartridge and not "heavy is the name of the game" in general.

I could be completely wrong and if I am somebody correct me. Hope this helps.

 

[gstaylorg]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

The manner in which the BCs and MVs of different projectiles influences how they are affected by wind is explained in exquisite detail and very clearly in terms that should be easy for most readers to comprehend in Bryan's book, "Applied Ballistics for Long-Range Shooting". If you don't have a copy, get one. It'll set you back about the same price as a couple boxes of ammo and the knowledge/information it contains will last far longer than the ammo:

http://www.appliedballisticsllc.com/Book.htm

 

[DCR]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Can't you just run a few typical set ups through balistic software and come to a close approximation on which one would suit your needs better?

From Shooter: 175 Matchking vs 155 Palma

Sierra 175 - 2650 fps- 10 mph - 1000 yards - 101.9 inches drift - TOF 1.711
Sierra 155 - 2950 fps- 10 mph - 1000 yards - 94 inches drift - TOF 1.55

Slight advantage to the 155 Palma but not much.

 

[earthquake]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

My next purchase will be Mr. Litz's applied ballistics book. From just the brief scan of it I got yesterday, I was blown away...so much great information in there. I had a hard time giving it back to my coworker!

I guess the defining issue here is my barrel length, 20". I know I'll never know until I do my own tests, but for curiosity's sake, how fast are people pushing 175 SMK's out of 20" barrels? Have those who've switched, noticed much more performance-wise, from the 155 Scenar?

 

[DCR]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I'm getting around 2550 with 178 AMAX from my Remington 700 AAC-SD. I've seen some of the guys with custom builds getting in the 2600's with theirs.

 

[Pusher591]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I get around 2530-2535 with FGMM 175 from a 17.5" Brux barrel.

McCourt Munitions 175 SMK load runs around 2545-2550 from the same barrel.

Both these loads were shot over 2 different chrono's with same results.

Both loads are sub to sub 1/2 MOA out to 400-500 yards. I thought your load might be a little slow, I have seen many custom 20" barrels in the mid 2600 range. Out of curiosity, are you reloading and what is your load if you are. Most loads will have 2 or 3 accuracy nodes, you may still have room to move up.

 

[nfoley]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I get about 2550 with my 20" barrel and 175 SMKs. Above that, using Varget, pressure signs begin to appear. Have been thinking about trying MR2000 to see if velocity is better. With same bullets and loads I get 2700fps with my savage 24" barrel.

 

[Tx_Flyboy]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Nfoley,

2000MR powder IS better than varget & will get u faster velocities at the same pressures. It also meters better, burns cleaner, and is just as temperature insensitive.

I haven't completed my load development, but so far, i estimate 2660fps from a 20" bbl.

 

[shooter.dm]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I get around 3000 fps with 155 hybrid target(bc .483) in a 24 in on my Howa

 

[Bob 964]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

SMK 175 G7 BC = 0.243
Scenar 155 G7 BC = 0.236

A drawback to relying on ballistics coefficients is that ballistic coefficients combine the effects of mass and drag into one number, so one bullet may have a higher ballistics coefficient than another simply because it is a heavier bullet.

In a May 16, 2011 Berger Bulletin, Bryan Litz wrote about the concept of “form factor” and how it could be used to analyze the efficiency and performance of a given bullet. The form factor is the sectional density of a bullet divided by its G7 ballistic coefficient. The G7 ballistic coefficient is used instead of the G1 because the G7 standard is a better match for the longer boattail type bullets most of us shoot, and should be more constant over a wider range of velocities, as compared to the G1.

The form factor measures how efficiently a bullet flies (drag), independent of its weight. Form factors of 1.000 or greater are bad; form factors of 0.999 to 0.950 are better; and form factors of 0.950 and below are the best.

Doing the math for the Lapua 155 Scenars (.308): 0.233 (sectional density) / 0.236 (G7 BC) = 0.987 (form factor). This means the drag of the 155 Scenar is less than the drag for the G7 standard. It is one of the more efficient .308 bullets.

Doing the math for the SMK 175 (.308): 0.264 (sectional density) / 0.243 (G7 BC) = 1.086 (form factor). This means the drag of the SMK 175 is more than the drag for the G7 standard. It does not fly as efficiently as the 155 Scenar.

 

[TiroFijo]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

squarenut, the form factor concept is as old as BCs...

The thing is, BOTH bullet shape and mass are important to retain velocity, and both are taken into account in the BC. The Higher BC bullet retains velocity better by definition.

The higher form factor value of the scenar simply means it has a more aerodinamic shape, but it is the BC that tells the complete story.

The only way to compare bullet performance downrange is using their BCs and reasonable velocities for both.

 

[Bob 964]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Tiro --- as you know, you can't separate one from the other. By definition, BC is equal to the sectional density of a bullet divided by its form factor. Sectional density, BC, and form factor all affect how a bullet flies and performs.

 

[Aimsmall55]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

I did a test out of my gap crusader with a 23" bartlein. The loads were as follows. 44.5 varget, lapua brass, rem 9 1/2 primer at 2.825" at 2740 fps. At 500 yards my come up was 46 clicks (1/4) zeroed at 100. 6-7 mph wind only 2 clicks adjustment. Shot 1/4 moa. Waited 20 minutes. Then rezeroed for the 155 scenars which were running 2883 fps. At 500 yards come up was 38 clicks and 5 clicks horizontally. Shot just as good but more Windage adjustments. Just food for thought

 

[earthquake]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

If I'm reading you right, 46-clicks and I think you're saying 1/4 MOA per click would translate to 11.5 MOA or 3.3 Mil elevation and 0.5 MOA or 0.1 mil at 500 for what bullet?

Then ~9.5 MOA or ~2.8 Mil for the 155's, but 1.3 MOA or 0.4 Mil wind?

Interesting. Yesterday at 1,040 yards I was holding 3.9 mils (13.4 MOA) of right wind with my 175 SMK's at 2,550 fps MV. Wind was full value at about 14.5 mph gusting to 20 mph.

Thanks for the info!

 

[George63]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: COfox</div><div class="ubbcode-body">

I guess the defining issue here is my barrel length, 20". I know I'll never know until I do my own tests, but for curiosity's sake, how fast are people pushing 175 SMK's out of 20" barrels? Have those who've switched, noticed much more performance-wise, from the 155 Scenar?

</div></div>

with a 20 inch barrel almost certainly the 175 will out perform the 155 - the palma concept: is long - to 30 inches and over (often tight) barrels with high pressure loads(the reason some try small primers to increase brass life) for high mv to wring out every last drop of performance from the 155 - without all those aspects the advantage ( and ease) will go to the 175

 

[Fred_C_Dobbs]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bryan Litz</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mkollman74</div><div class="ubbcode-body">I am looking into this as well. I haven't fully processed some new info I found, but it is intriguing. Litz wrote an article where he addresses the BC vs speed issue. He states that for every 0.1 in lower BC, you need 496 fps to make up for it. Not yet sure if that applies to G1, G7, or both. Anyhow, it's a good and practical ratio to keep in mind. </div></div>

Someone please tell me what article the above statement is from.

Thank you,
-Bryan </div></div>
Bryan, that's in part 2 of the article, "Understanding Long Range Bullets", posted on Applied Ballistics (page 3, first paragraph):

"As a general rule, you can go about 496 fps slower for every +0.100 counts of BC, and match wind deflection."

 

[BryanLitz]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Thank you. I totally forgot about that part of the article. As with many technical 'rules of thumb', it's important to consider the caveats. The very next paragraph of that article reads:

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">"To echo last month: Most of the calculations in these articles are not intended to show hair splitting precision, but rather to illuminate some basic trends, and approximate magnitudes. Having an educated intuition and some useful rules of thumb can be helpful when youíre trying to make decisions
involving so many complex variables. <span style="text-decoration: underline"><span style="font-weight: bold">In other words, don't hold me to it; it's just an estimate.</span></span></div></div>

Caveats are important.

* The rule of thumb being repeated is only valid for the specific illustration, and is different for different ranges (other than 1000 yards).
* Furthermore, the equivalence only applies to wind deflection, nothing else like drop.
* Finally, it only applies for G1 BC's, which are less relevant than G7 BC's for long range calculations.

-Bryan

 

[bubbapug1]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Has anyone loaded up both bullets and shot them out to 1000 yards? A bullet can look great at 800, even 900 yards, and start to tumble badly and never arrive at 1000 consistantly.

 

[Ol' RD]

Re: BC/Performance Question: 155 Sc. Vs. 175 SMK?

Both, bullets will get you to 1K without to much trouble. Read Mr. Litz statement again. Cause there's a ton of info in his second post you "Might" want to pick up on.