.308 168gr N140 questions

[bewsh]

I have just started loading for a .308 target rifle and due to availability and most regular range (600 or below) I plumped for the 168gr SMK

 

its a 29" barrelled monster but I was wondering if anyone more knowledgeable about powder choice for long barrels would be able to comment.

 

I worked up a load using N140 (Norma Brass, CCI200, starting 50 thou off lands)

There wasn't much data online for the combo but I found one load that gave me an obvious accuracy node at 43.6gr

(no pressure signs either here or the increments up to 0.6gr above)

above and below it opened up to 1/2"-1" C2C so it was pretty obvious

 

I was a bit rushed and only used three shots groups (yes I know I should use more) but I am pretty happy..

 

 

it was chucking it down with snow so I couldn't run them over the chrono

but I got someone to run it through Quickload

 

43.6 N140
55 kpsi
2746 fps

 

 

Question is:

the chap with quikload suggested it was too hot pressure curve wise and a slower powder might be better for barrel life and recoil (unlikely to see huge volumes and given the weight of the beast is barely an issue)

 

Is this a load or even combination anyone else uses as a target load?

 

I have always wondered what N160 might be like in long barrel.

too slow? (I have a lot of it is why I ask!)

[Jagged 77]

I use N140 / 168 SMK's out of one of my .308's with a 20" barrel - it is extremely accurate at short range (up to 300 yards) then performance drops off. The velocity is a pitiful 2500 fps but that's where the accuracy node is. Out of another I use N150 and 167 Scenars with much better velocity.

 

N150 is worth a look at especially with your longer barrel and is the powder to go for (out of the single base Viht offerings...). N160 is just too slow. The general consensus is that the 168's hit a brick wall at 800 yards, therefore if you want to push out further research some of the threads on here over the last year on .308 bullet choice, there's some good info already written.

[Laurie]

Find a more knowledgeable friend! As his QuickLOAD run suggests, this is not a high pressure combination, and N140 works fine with the 168gn SMK. If you need to set the jump at 50 thou' (why?), it suggest there is a lot of freebore there and you won't achieve 55,000 psi or the computed MV. 15 thou' is a sensible amount of jump fopr this bullet.

 

I'd personally prefer Viht N150 with the 168 - it usually gives superb results - but if N140 groups well stick with it. N160 would be a terrible choice.

[bewsh]

thanks for that

 

I think it was the curve rather than the limit that he was commenting on, not sure how that is expressed in QL

 

The MV if correct, seems very good for my purposes, (rough estimate of 2700fps says it is even supersonic at 1050yds!)

will see what it does at 500 on Sunday though

 

The jump is not so much a "need" as just happened to be where I started to get going.

(bit rushed this time round, total fluke I got this group after the cluster@@k i had to get zeroed and on paper, with a rifle I have never fired, a scope that wasn't zeroed and no other ammo than the small group charge level batches!! not idea!)

 

Not sure I can shoot well enough to demonstrate or justify a longer OAL to be honest! (assuming I can replicate what I just did in either load development or shooting!)

 

It feeds well as it is and the longer dummy rounds i have of 175, 190 and 208gr were a bit snaggy coming out of the magwell

(push feed drop in feeding is not an option as it's a legacy Military Mauser, controlled feed, fixed bottom metal plate/magwell, rebarrelled/regulated by Fultons of Bisley with a Lothar Walthar barrel)

 

will see how we get on before I look at slower powders

[Laurie]

(rough estimate of 2700fps says it is even supersonic at 1050yds!)

 

 

 

'fraid not. Firstly, you only get anywhere near that sort of result using G1 BC values which are poorly aligned to HPBT match bullets. The correct type of BC to use for long-range ballistic calculations and this shape of bullet is the G7 which is based on a 'reference projectile' whose shape is somewhat akin to the bullet being used. (Think of the shape of the standard velocity 40gn RN solid used in .22LR match ammunition and that's what the G1 reference looks like.)

 

Bryan Litz gives an average G7 BC of 0.218 for the 168gn SMK. Run that at 2,700 fps in standard ballistic conditions and it calculates 1,050 at 1,000 yards, barely supersonic. However, that's a flattering figure as this 168 was devised aroyund 60 years ago as a bullet for 300 metre ISSF 3-position shooting and is optimised for short-range performance. A long-range bullet wants a 7-9 degree boattail angle; the 168 SMK has 13-deg, far too steep. It causes the airflow to separate from the bullet walls during transonic speeds causing severe drag and worse, turbulence. The result is that the average BC is in fact too high to reflect its performance at distances beyond 800 yards. 1,000 yard shooting with the 168gn SMK usually sees very poor accuracy and signs of instability on the paper. It's fine for 600 yards though, although there are lots of designs around with higher BCs. At up to 600, precision (group size) generally counts for more than external ballistics - within reason anyway - and a good 168 SMK load is usually easy to tune and groups very well.

 

You may not be getting 2,700 fps plus anyway. The Norma case is roomy compared to Lapua or RWS which reduces pressures for any given load (useful for getting a little more MV at max loads / pressures though). If your chamber freebore is such that you can use 190s (make?) and the 208gn Hornady without them being pushed half way down the case, pressures and velocities will be severely reduced needing appropriately higher charges to restore them. (Again, as with roomy cases, a long freebore will if not taken to excess allow ultimately higher MVs within safe pressures as it increases the combustion chamber size.)

 

FWIW, I carried out a full series of large rifle primer tests last year in my FTR rifle which has a 32-inch 5R Bartlein barrel chambered to suit the 185gn Berger Juggernaut and too short-throated for the 208gn A-Max. Using heavier / lower capacity RWS brass, the antediluvian 168gn Hornady HPBT 'National Match', a bullet with very similar characteristics to the Sierra, the average MV with the CCI-200 was 2,795 from 44gn N140. This was right on the QuickLOAD prediction. Reduce that to 43.5gn charge weight, knock three inches off the barrel, add 1gn of water capacity for the Norma case and a bit of extra freebore and you lose over 100 fps. You might still be there though given Lothar Walther barrels being on the tight side and 2,700 is sensible in calculating the required elevation come-up from a 100yd zero - 11 3/4 MOA using a G7 program.

 

I'm intrigued by your friend's analysis of the pressure curve shape. Run this set-up with N140 followed by slower burning N150 and taking them up to the same peak velocities and you don't see any discernable difference ion curve shapes. What QuicklOAD does provide, but is never commented upon is a thermal efficiency calculation. Your load is rated at 35.7% which is a very good result. Move to N150 and increase the charge to provide the same peak pressure and you get 33.6%, a ballisically less efficient combination. QL is very good at allowing an informed choice of propellants that will likely work well, giving ballpark results, and saving a lot of time components and barrel life in load development, but I remain dubious about many of the claims that say if factor X is near to, far from, the same as factor Y it'll be a killer load. In my experience the program is not a good predictor of what produces small groups as these are so often dictated by small differences in indidivual components and firearms specifications.

 

 

[bewsh]

Awesome as usual!

 

can't comment on the chaps QL interpretation having never seen or used it

 

The 1000yd thing was aspirational at best

I have read enough about the 168 SMK to be put off even trying it at that range!

I need all the help I can get so starting with a brick is not a good start.

I will be doing some with 190gr first and if I can bring myself to tax my 300WM of its favourite 208gr fodder..those too.

 

I have a set of dummy rounds I made for all possible choices, seating them to the lands for each round (below)

 

From the top

PPU 150gr FMJ

168gr SMK

175gr SMK

190gr SMK (all same OAL as same Ogive which makes me think the snagging was not OAL related)

208gr AMax (with round to indicate depth in case)

 

 

 

 

Does that look deep? I thought it seemed OK at the time, boat-tail is barely below the neck

 

 

Your G7 drop calculation is interesting

My cheapie BulletDrop+ on my phone puts it at 11.4 and 11.7 MOA for both G1 and G7

(based on 2700fps, 100yd zero, G1 0.462/G7 0.218 Alt:419ft Air:7degC, Humidity:78%humidity)

[Brillo]

I've used the 168 SMK with 44.0 grains of N140 with real success in my Sako TRG-22, which crono'd at +2700 fps and very tight groups up to 600 yards. Reloder 15 also proved to be very good with this bullet.

 

I actually ran this bullet/powder combination as a series of tests for a friend who was going to be shooting long range for the first time and all he had was the 168 SMK, and they were all seated to 2.800" COAL to ensure he could chamber them in his the AI AW. Like I said, up to 600 yards performance was excellent but when he shot at 900 and 1000 yards the bullets tumbled and scattered all over the place. That's when we learned more about that bullet's particular ballistics.