Disappointing 308 MV.......and......which powder in Lapua factory 308?

[Brown Dog]

Been disgruntled with the MV of my pet 308 load ever since I read posts here claiming 180gr bullets being pushed accurately out of 308s at 2700fps.

My pet 308 load only manages to push a 165gr NBT at 2580fps.

Accuracy/consistency is stunning (0.25moa or less)but I'm disappointed with the MV.

I load 45gr of N-140 under a 165NBT. (Viht tables suggest I should get 2670fps at that charge weight)

Any ideas why I'm being short-changed by 100fps?

-I'm having trouble visualising how the internal ballistics of my rifle could be wasting that much performance. Be grateful if any of you could indicate whether your experiences have led you to consider 100fps 'slow' to be par-for-the-course or 'within spec'?




.....Also, a while back I had calculated the charge weight of Lapua factory 167 scenars by weighing loaded rounds and comparing to fired cases -didn't want to waste a round!- they are also 45gr. Yet they claim 2690fps (never chrono'd them)

Been trying to account for the difference, curiosity just got the better of me and I just inspected the contents of a factory Lapua 167 scenar round. I'd assumed since -forever- that it would be full of Viht 140.

Granules were noticeably larger than N-140. Anyone know what they use?



Grateful for any answers. Meanwhile, back to loading some more of my weedy pet load -almost seems inhumane /ubbthreads/images/graemlins/smile.gif

 

[308sako]

B Dog, I don't know what Lapua uses in their Factory rounds, but I have to agree with the chrono results you get. My pet loads rarely exceed the high 2500's with 165 Nosler BT's and they shoot great and kill great so why worry? Not unlike years ago when cheap chrono's were rare items at the local range, perhaps chrono's haven't hit the market so well in Europe... Even using a stiff load of VV540 I am not able to come up to book max or the velocities claimed in one of my .308's. Enjoy this great round and don't worry about small velocity differences.

 

[James Jones]

Some guns just won't shoot accuratly with high pressure , you may have to play around with differant powders
I have a couple 308s that don't like high pressure and two that shoot better the hotter you run them
But in all of them I've been having great results with VV
N-550 , higher velocities and lower SD averages.
You may want to try a differant bullet like the 175gr SMK , a couple of my guns like those better than the 165gr class bullets and one like the lighter ones better , for that gun its gonna get a steady diet of 150gr Accubonds from now on as they shoot great and hold together well.

 

[Brown Dog]

308sako and JDJones -Both,
Thanks.

I'm going to make myself sound stupid here:

My MV of 2580 was calculated (not chrono'd) based on field observation (ie actual comeups) and fitting those observations to my ballistics program.

Using the viht data for a 45gr charge of V140 (2670fps) and Nosler's stated BC (0.475 for the 165 NBT) as a start point, I 'massaged' the BC and/or the MV in the program until I got a match for my field observations.

Massaging the BC would not produce a fit. However, dropping the 'assumed' MV to 2580fps gave a perfect fit. I therefore took it that my MV was only 2580fps.

Due to the frustration indicated in my first post on this, I borrowed a chrono.
Today I clocked my rounds. They're doing 2740fps 10ft from the muzzle.

I'm pleased they're going faster than I thought, but can't understand why I'm having to do a 160fps MV 'fudge' in order to ballistically model the trajectory. (The program is based on the Pejsa model and validates when compared to data from other programs or –even- the Federal ammo catalogue data.)

I'm confused. Any ideas whats going on here?

 

[nowler]

hi matt

a couple of points of interest or observation for you.

1. my AI would not shoot high pressure loads well. i could not get it to settle in the higher tuning node at all. i suspect that maybe all the lugs weren't bearing equally but i am not sure about this....

2. if your load is accurate, then why change it? if it ain't broke.....

3. your ballistics program could be well out. are you using accurate temp, humidity, altitude, air pressure readings etc. etc. these can make the difference. also, what kind of chrono was it as this can make a serious difference sometimes. you could check the chrono by firing some .22 subsonic over it to see what they come in at.

4. velocity is well over-rated. 200fps is not gonna kill anything any more dead imho. accuracy is the way to go over velocity any day. most of my loads are not super hot and i have no complaints.

derek

 

[308sako]

B Dog, I am glad that you now know what velocity your loads are achieving in your rifle ( and jealous) But perhaps some additional shooting at extended ranges and some observations about barametrics, and elvation above sea level. Also it would be much easier if you had been shooting the Sierra 168's or 175's as there are loads of highpower shooters who have all the data you could want. If you have access to a range where you can shoot at 100, 200, 300, 400, 500, 600 yards record the results. Also I would stress that you accurately measure the sight height above the bore. Ultimately this is why the pro snipers keep a log for all shots fired and conditions under which they were loosed.

 

[Brown Dog]

Derek, 308 Sako,

As suggested, went out to check the chrono's calibration today. Not going for statistical validity here, trying to get a feel for what's going on:

2 rounds of Lapua factory averaged 2689fps. Not bad for a claimed MV of 2690!!



…….Chrono calibration confirmed.

This was the set-up, chrono 10ft from muzzle. It's a CED Millenium chrono:


My pet load clocked 2722fps (the cases I used yesterday were 'spares' rather than my super-duper prepped cases):



Despite confiming the 2722fps actual MV, the data input required to model the 'observed' trajectory is as follows: /ubbthreads/images/graemlins/confused.gif

MV (fps) 2580
Bullet Wt (Gn) 165
BC 0.475
Start Range (m) 0m 0 Yds
Increment (m) 25m 27.34 Yds
Impact Ht (cm) 1cm 0.3937 in
Zero Range (m) 100m 109.36 Yds
Pressure (mbar) 1000
Scope Ht. (cm) 4.9cm 1.91 in
MOA value @ 100 yds 1.047in
******. Coeff. rate 0.5
Break Velocity (fps) 1300
Ammo/Rifle Description AW: 165 BT, 45 gr V140, 56.7mm

Laser Range: 101 Angle of Sight: 0
Wind Speed: 15 Altitude (m): 115 Pressure (mbar) 1000
Wind Dn: 3 Temp: 11

By the way, 46grs of v140 pushed it up to 2800fps. Reckon that'll be my Scotland load /ubbthreads/images/graemlins/smile.gif …but what MV will I input to model it??? /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/confused.gif!!:

 

[nowler]

hi matt

if your atmospheric conditions are measured accurately, and you can get me your height above sea-level, i'll plug it all into exbal for you and send you a drop table..... if you could get to a range and measure the actual drops further out, i could plug them in and it would adjust it accordinlgy for you.....

cheers

derek

ps, where did you get your aci and how much was it?

ta

 

[Brown Dog]

Derek,

Thanks, I've spent all afternoon mulling this over in the back of my mind, waiting for the flash of inspiration.

Just spent 1hr fudging every variable (given a fixed 2722fps mv rather than the fudged 2580 I've been using) tried messing with pressure, alt, scope height, ****** value, BC etc /ubbthreads/images/graemlins/crazy.gif in order to try to match field observations. No success, only 2580 mv puts me in the parish /ubbthreads/images/graemlins/mad.gif

Until, literally 10min ago, it occurred to me that the S&B might not be giving me spot on 10mm clicks. What if they were, say, 9mm clicks?

I just input a formula that divides my 1cm click values by 0.9, and lo and behold, at an mv of 2722 I am firmly in the parish! 0.88 seems closer yet.

This will need a bit more investigation, but I think this may be EUREKA

PS I'll drop you an email regarding the ACI

 

[Brown Dog]

Derek,

Popped out yesterday just for a check-sanity plink. Didn't have the space to put up a long range target, but stuck one up at 134m and another at 248m. Dreadful condition, wind all over the place; couldn't get under 0.5moa but, my comeups were on the nose. So.....here's the weird bit:

Previously, to match my field trajectory observations, I plugged 2580fps with a scope click value of 1cm into my ballistics prog,

Now that I know the rounds are actually doing 2722 there is no 'normal' data fudge (ie BC changes etc.) that will allow predicted data to match my field observations. But..

If I plug in the actual MV of 2722fps but with 0.87cm clicks, the predicted data does match my field observations!:

The comeup values remain within half a click out to 700m!

.....Now I have to think of a way of measuring my scope click values to confirm this solution /ubbthreads/images/graemlins/frown.gif

 

[rost495]

Re: Disappointing 308 MV.......and......which powder in Lapua factory

Watching and reading this thread.

I was as involved as you are on a bit of a different subject. Let me try to help save your sanity.

I am being totally serious. It was suggested before also. Just go out and record conditions and record the actual impacts at distance.

Who cares why what how etc...... There are going to be so many variables involved it'll drive you nuts. And there is no reason to wonder.

The one thing I would have to know is the clicks on the scope though. Just do a few tests to figure it out. NOT how it matches your data so much as how much physical movement it gives.

Other than this, life is too short, Jeff

 

[Brown Dog]

Re: Disappointing 308 MV.......and......which powder in Lapua factory

Jeff,

Wise words!

Trouble is, because I'm afflicted with 'ballistics-nerditry', I have to know! /ubbthreads/images/graemlins/smile.gif

Checked the tracking on the scope. It's giving spot on 1cm clicks. Maybe the 'fudge' I was looking for to match real world to program modelling was simply to input a false scope click value -time will tell!

 

[nowler]

Re: Disappointing 308 MV.......and......which powder in Lapua factory

maybe the ballistics program is using the wrong drag function for the style of bullet. a g1 is the norm but does not represent boat tails all that well. hopefully someone who know more than me will chip in here?

how accurate is your chrono too? have you tried testing it with subsonic match ammo to see if it is in the ball park?

come back to me about your trajectories, were they in metres or yards? is the drops in inches or cm, and are your adjustments on the scope in moa, cm, inches or clicks?

cheers

delboy

 

[Brown Dog]

Re: Disappointing 308 MV.......and......which powder in Lapua factory

Hi Derek,

I'll blip you an email.

I 'calibrated' the chrono with the known value Lapua factory ammo shown above; it clocked spot on the factory claimed mv.

I've played with all the drag functions and ****** coefficients. No combinations model the trajectory accurately.

The only fudges that get me in the parish are: Fudge MV to 2580; or fudge scope clicks to 0.87cm