True MV way below load data

[Napalmio]

Hi folks,

 

With my .308 reloads, the real world MV that I get for a given charge differs substantially from the MV in the load data.

 

For example, my .308 load uses 44.2gr of TR140 in Lapua brass, under SMK 2155 heads. According to Vihtavouri data (for N140 and closely matching TR140 data I got from Shooting Shed's website), That should produce around 2700fps and is Viht's max charge. However, when I check with a chrony I get 2385fps average +/- 2 and when I measure actual drop at 200, 300 and 400 yards it correlates to 2380fps in Shooter, so my two real world measurements absolutely agree.

 

Can anyone explain why I get such a low MV for that powder / case / bullet combo compared to the load data? I appreciate I have a 20" barrel and the Viht data is based on 24", but that should only reduce MV by about 80fps at worst from what I can find in tests online and I'm 315fps low.

 

As per my previous question about a random MV outlier, I'm not trying to resolve an accuracy issue, the load is an absolute beauty in my modest rifle and performs far better than I can exploit given my poor technique, I'm super happy with the load but i'd just like to understand better why I get such different velocity results compared to Viht's data.

 

Any ideas?

 

Cheers,

Mick

[duey]

Hi Mick

You won't match the published data as your comparing apples with pears not apples with apples

 

I believe from your other posts that you are building longer than SAMMi specs therefore you will have reduced pressure for same powder weight due to greater available volume in the case

 

If you used the same brass, same primers, same bullets, all built to SAMMI length there's still no guarantee you will match their data as you're using a different rifle

 

I believe you will need to be loading in the region of 46grs of powder to match published data velocities

 

Published data is a very good source of info as is things like quick load, don't ignore it and the limits they set out, but real world data will always differ

Even different batches of same maufacturers powder, bullets, brass, and primers can differ and effect pressures and velocities

 

Then of course there is atmospherics to consider, temperature, humidity and elevation etc etc etc

 

So use the data that's out there as starting points or reference points and proceed carefully when and if you venture beyond their recommendations but remember your bullet is different to theirs

 

Keep bashing on bud

 

Duey

[VarmLR]

As above, real world velocities seldom match published load data for the reasons already given.

 

According to load data, I should be seeing close to 2710fps using 43.1g N140 for my 150gr bullets. Measured velocity from a 10 shot string averaged 2650fps seating to 2.75" COL from a 24 inch barrel. My brass and barrel and chamber will all differ from the test barrel used for the load data, so I'm not surprised that figures don't match.

 

I haven't yet tested my new batch of powder to see if there's any variation due to batch differences, but I should really check on my next range outing as keeping records of things like this can be invaluable later on when trying to figure out possible causes of any encountered inconsistencies.

 

Finding that your real world MVs don't match load data isn't something to be overly concerned about unless you're loading for longer distances and need to keep the bullet above transonic velocities at your chosen range.

 

Then there's things like temperature and temperature sensitivity of the powder used to consider along with other factors.

 

Load data is really only a guide, and a helpful one too. Once you have worked up a safe load which you are happy with for terminal ballistics and accuracy, this becomes your real world load data. I had some very helpful assistance on UKV when trying to develop a load for my 223, and was given some QL data kindly worked up for me. It was significantly different to tested results which can only be down to the differences between predicted barrel pressures and actual, possibly due to my barrels being a lot more polished/slippery (!) than the simulated barrel assumptions resulting in lower actual pressures. My measured MVs were a fair bit lower which backs that up, but the data provided helped me into the right ballpark which is what it's designed to do.

 

I believe that there may be ways within QL for example to recalibrate certain model data, so it should be possible to more closely simulate your own rifle's performance should you require new load data. To some, that may be of more use than to others.

[Swarovski1]

My 6.5x47 using 39grs of RS60 and 140gr amax is going 2900 fps.way faster than load data.by 300 fps I think.my batrel is 3 inches longer than data barrel.not sure off hand the oal is but it's 10 thousandths off the rifling.i plan to double check with another chronograph but to date I've had no problem with it.i did doubt it last year as my mv went down by 100fps in my 6.5 06.i upped the powder charge and I was back up to speed.

[Napalmio]

Thanks guys, your input is a great help. It's interesting that I have previously loaded with Varget, when I could get it reliably, and the true MV was very close to the data, whereas the TR140 is so short. I'll go back through my notes to see what charge I was using with Varget as Duey has now got me thinking... My 223 load uses TAC and is spot on compared to the data too, so it seems that only the Vihtavouri data leads me astray. I'm going to search UKV for all the info I can get on reading pressure signs then maybe go through the development process again, although the chances of me going beyond 800 yards is very slim so what I've got is perfectly good enough. But then, 90% of the fun is in the development and experimentation, so stuff it, "I'm going in!"

[Chris-NZ]

The biggest variable not yet mentioned is bore dimensions. It's quite possible your particular barrel has rather loose dimensions and this is not conducive to good vels.

 

The loaded OAL length is not a variable of significance to pressure. If you think about it, once the projectile is underway in the barrel, the enclosed "pressure vessel" volume is hugely bigger than the case volume.

[ejg223]

The biggest variable not yet mentioned is bore dimensions. It's quite possible your particular barrel has rather loose dimensions and this is not conducive to good vels.

 

The loaded OAL length is not a variable of significance to pressure. If you think about it, once the projectile is underway in the barrel, the enclosed "pressure vessel" volume is hugely bigger than the case volume.

 

That would be my guess also.

At the moment I am comparing a T3xCTR with 20" that produces 2600 fps with 168gr Hornady Match to my 22" Lilja barrel also in a T3 action that achieves a tad over 2700fps. Hornady box say's 2700fps.

My previous tight bore Bartlein Palma Barrel also had higher than normal speeds and pressure.

 

My now sold Rem 223 20" produced 2950fps with Fiocci 50gr v-max, my T3 20" only gets 2840 fps from the same box...both well below the 3300 fps claim on the box. Also if one takes barrel length in account.

My 243 T3 22" 2900fps vs 3066 on the box with 90gr Gamehead, 70gr. Arrowhead Sako is also way down.

edi

[gbal]

This is a another puzzler-several possible general factors have been put foward,and VarmLR's advice about the fit between book velocities and real world/your rifle is very well discussed.

As earlier post,a single low chrono freading might be a blip by the chrono,or just a low powder charge gone un-noticed at loading. We can never know,but repeated firing with more ammo loaded as before usually eliminates one explanation....(or both!).

 

Here something more complex is going on.

On a string of shots,the chrono readings wee consistently low,compared to the 'book' values for that load,viz: book 2700,chrono 2385 +/- 2 fps Charge was 42.2 TR140

 

Suppose we allow 100 fps for the 4 inch shorter barrel-that still leaves a very considerable 200 fps or so under 'expectations',which is probably too much to be acdelightful chapable by atmospherics etc.

 

This load is an 'absolute beauty'-which I take to include accuracy (it should be fairly mild shooting,too!)

 

Note too,Varget loads were 'very close to the data'-ie the book velocities-I find that difficult to square with a bore issue.

(and probably any systematic reloading error -223 with TAC also 'spot on with the data"-and a fps variation of =/- 2 fps is exceptionally consistent- so very unlikely a powder underthrow-though it might have been,I'll assume it isn't below)

 

Note the issue is not the 'conservative' Vihtavuori data-rather that this conservative data is not replicated by actual firing (whether there is any real gain in upping the load is an accuracy issue,we can't decide here).

 

So we have an accurate load,which is slow compared to 'book'.

One possibility is the powder is not up to snuff,eg a 'mild' batch and/or deteriorated in storage.

 

Storage is checkable-Napalmio can tell us if anything differnt happened-eg there is a recent report in Accurate shooter about velocities changing when storage went from garage to house,and the were restored when storage reverted to garage,with humidity being implicated. (sort of 'keep your powder dry"-or at least consistent.(Probably diferrent powders resist humidity changes,somewhat as they do temperature changes-though doesn't seeemto apply here-if all powder stays in the reloading room.)

 

Batch issue.....well lets see...that is not unknown,of course-though 200 fps is on the high side...but what was actually loaded...it seems the load was 42.2 g of TR140...then the comment...

 

"should get 2700fps,as Vihtavuori's max charge max charge...but chrono gave only 2385 fps-according to the Vihtavuori data for N140 and closely matching TR140 data from SS website...."

 

This might be more clearly expressed,but it seems that V140 powder was used,and TR140 data compared?

Now,there probably was close similarity of the V140 powder and TR 140 powder ,but there is room for a possible diffference-at least in batch....,and the derivation of SS data isn't available .I mean no criticism,just that we don't really know about this.

 

So,can we exclude a lack of 'pop' in the powder used,or assume the comparisons are actually completely valid?

OR even that there is not some complex interaction going on with the individual rifle and powders,which is one of the general caveats,as per Varmlr's post,and repeated in others.

 

Remember,of course ,that the load is 'an absolute beauty' : let's hope it remains so (+/- 2 fps,anyhow :-)

 

gbal

[Napalmio]

Thanks again for the in-depth discussion. Just to clear up some questions, I was referring to Viht's N140 load data because there was a scarcity of any for TR140 when I developed this load, other than the figures that Shooting Shed posted on his blog, and I was of the understanding that TR140 was a clone of N140 so the data would be transferrable. However, I've just searched for Reload Swiss data having gone back and re-read the SS articles and can now see that since RS rebranded the TR powders (TR140 is now RS50), there is some data (albeit thin) on their website for Sierra 150gr (not 155gr but close) over RS50, which shows a much higher max charge of 47.4gr for 2717fps, so suddenly my 2385fps from 44.2gr looks entirely reasonable given the shorter barrel. It seems that this suggests that the Viht data for N140 that I was using as my baseline (and which does compare closely to Varget data with 44gr delivering a theoretical mv around the 2700fps mark) is indeed very different to the actual performance of TR140 / RS50. So, in essence, if I'm understanding this right, my basic mistake is believing that TR140 and N140 were one and the same from a performance perspective, based on consensus and in the absence of any data from the manufacturer at the time?

 

Storage wise, the powder is always kept in the same place, although temperature and humidity varies widely from winter to summer thanks to an ancient house with no insulation to speak of and damp cob walls.

 

The one "rogue" mv was 80fps higher, not lower. As I said, I will repeat the testing in more controlled conditions and with a greater number of rounds to see if it's a regular occurrance or was indeed a one-off.

 

Thanks again for all the input, I'm always impressed and humbled by the depth of knowledge (and patience!) on UKV.

[VarmLR]

RS powders may be similar, but they're not the same. Trials that I have seen (and read about) using RS52 as a comparison with V-N140 reveals that the RS powder has a few surprises up it's sleeve such as seemingly a higher velocity to pressure ratio plus is reputedly less temperature sensitive. I haven't seen any data for load V's pressure but I believe that it is more linear (less sensitive) than 140. In short, it ought, in 308, to deliver very consistent velocities for slightly lower pressures than N140.

 

I have tried N140 loaded to a not untypical 44.5gr with 155gr SMKs, but this showed clear pressure signs in my rifle, so I backed off and found the next best node for me was 43.1gr (2650 fps with 150gr SGKs). As I can't get consistent supplies of RS52 (yet) I continue to use N140 and consider it a very fine choice for .308 (and my 223 for 69g bullets).

[gunner]

Nap , just wondering how far off the lands you may be with your 308 load ? If your say 50 thou off then you can bet yer boots if you loaded 10 or 5 thou off ( with same charge ) the pressure / fps / trajectory etc would all be very different , but just a thought . Keep it safe as allways

[Swarovski1]

I used RS 40 with 155 grainers in my 308.i ran on 41.5grs.think they were doing around 2900 fps on a 26 inch barrel.155gr is max bullet weight in 308 . another fast powder is AA 2520.again I am thinking 45.5grs gave me 2900 fps with 155 grainer.if you want extra speed for short barrel use a double based powder.