velocity mismatch

[Miseryguts]

Hi, am working uploads for a 223, although the loads so far are accurate enough, I see a discrepancy between the MV quoted in the load tables and my measured MV, albeit with a fairly budget Chrono. I am seeing MV's 7% lower than quoted. Is this to be expected or am I doing something wrong in the reloading? I have the Chrono about 5 feet from the muzzle, and a good stiff crimp on the bullet. Velocity variation in a 10 shot string is around 60 fps, and Sd is 25

[MrCetrizine]

Does the test rifle from the load data match yours for barrel length and twist rate?

[The Gun Pimp]

Such 'tables' are only a guide - how can the table writer know anything about your barrel, brass or bullets you are using? All will affect MV.

[Miseryguts]

ok should have thought of all those possibilities, been a long hot day, mea culpa!!

[Cumbrian 1]

The MVs in reloading tables are overly optimistic. I have given up with chronographs, once I have developed a load I simply shoot a target at approx. 800 metres and concentrate on the number of elevation clicks to get on target (regardless of the targetdistance it must be known) record the temp. air pressure it is best to avoid extremes in temp and then work out my velocity backwards i.e. on my ballistic app I input the atmospheric data, twist, bullet G7 figure etc and the adjust the velocity until it matches the correct number of clicks for the test distance. I then test it at various ranges, I have found that this gets me the quickest results however I don't shoot competitions but the accuracy of my data is good enough for my needs.

 

By measuring the MV with a chrony is has never given me data which is any good and I have tried a few.

[VarmLR]

I use a pro-chrony which seems to give very consistent results, and compared back to back with the more expensive ones like the magnetospeeds, seems pretty much spot on. Entering my average velocities into Strelock and then shooting at 400yds usually gives pretty decent results, comparing the drop from 100yd zero. I rarely shoot vermin out past 400 yds hence this is good enough for me.

 

It is perfectly normal to see MVs lower than load tables suggest, as said, most seem very optimistic. MVs in my rifles are also lower than predicted from Q-Load; eg in 223, my 60g Vmax load using 23.1g N133 give 3150fps (measured).

 

It's very rifle/load dependant. Main thing is consistency. Must admit, I never ever bother with crimps for a bolt action, especially since many of my loads are either lightly compressed or approaching full as I am concerned about raising pressures too much. Plenty do use them though.

[Laurie]

Loads pressure data measurement and the resulting starting / max charge weights are usually done in SAAMI or CIP spec barrels with the chamber and freebore likewise for that cartridge. Some of the US companies then use a factory rifle for the MVs bit as an extra test to produce more representative MVs as it was found that MVs from factory rifles and those from the industry standard barrel in a universal receiver bore little relationship.

 

That's because factory rifles often have 'generous' bore / groove dimensions, and in many cases have massively 'generous' freebores. Remingtons are notorious these days for this and many out of the box .308 Win Rem 700 PSSs and suchlike are so generously 'throated' they will accept very long match bullets such as the 185 to 215gn Berger Hybrids seated well out, and shorter / lighter bullets can't be seated anywhere near the start of the rifling.

 

Throw in a large 'slack' chamber too at the maximum SAAMI tolerance, and all this can reduce pressures and hence velocities massively, so much so that some maximum charge weights in some handloading manuals are really just a joke in many rifles. The problem then is knowing what really should be applied and what can be safely ignored, and that's where it's wise to treat the manual data as a good guide to start load development, but let the rifle and the brass tell you what is happening. That in turn takes some experience or assistance from one with it.

 

A chronograph is IMHO a near essential tool these days for the handloader who wishes to get anywhere near maximum and/or optimum performance, and/or wants to shoot at long ranges with cartridges like the 308.

[gbal]

The issue of a mismatch between reloading tables and chrono data-and firing data measures- comes up repeatedly;as does the perhaps related mismatch of the popular Quickload tyoe guides.

 

MRC,Vince (GP) factors that help explain discrepancies......in general terms,the issue is that the data input into the tables/ballistic soltion/QL/etc can differ from any actual real world input for a specific rifle and load under particular conditions,even assuming accurate measurement (not a given with some chronos ,eg).

This is a very complex issue,with many factors potentially varying and affecting the results-and some factors are not linear in their effect (simply put,a variable-say tempersture-can have more effect in one cartridge,or barrel length,or ambient te,perstaure that it has at differnt values on any/all these-they may even interact and interact variably.......

Si it is fairly easy to get a rough match on most variables,but it's very difficult to get them all lined up with the base conditions in the tables/programs.....Laurie gives the example of SAAMI -used by official labs,but there is tolerances in manufacture so chambering and leade etc can vary,and these affect pressure,and velocity....

NOTE-I don't think any home reloader can measure pressure directly-and pressure signs are only that-symptomatic:they may be absent,despite pressures for example (Palma brass shows less than regular 308 eg).

 

But there are other variables-some of which are not systematically included.Let's consider just one such:

 

WE all know temperature matters-loads are higher pressure on atmospherically warmer days,probably an effect on the powder. (some keep ammo cool for that reason).Well supported (very approx.ball park,1/10 g powder increase is similar to 3F degrees warmer).

 

BUT,who allows for barrel temperature as they shoot strings of shots in load development.....hmmmm-no-one? does it matter?

 

Well,yes probably-see referenced research below for details: basically,it was found that there was a pressure rise of 204psi per Fahrenheit degree of barrel temperaure increase Ithis is specific to powder,cartridge and temperature range-eg for 4350 in the same case it was only 177psi per degree.

That is considerable-at barrel temperature of 65 F,pressure was 5,200 but by 75F it ws about 5.4,and by 90F it was 5.8 (thou psi). Note Varget is rightly considered quite temperature insensitive.....

.....one possibility might be that the heating mass of barrel/receiver etc acts to heat the primer-but that is just a speculation. The important finding is that barrel temperture matters,and increases pressure,and is not the same factor as ambient temperature,or powder temperature....AND it is not incorporated in tables/ballistic calculatots etc etc (and of course different barrels heat at variable rates-as is well known-even with the same ammo...).But we don't use that information in calculations.

 

OK the pressure differences for plausible realistic barrel heating are enough to vary bullet performance etc,so become an unknown in trying to match up tables etc with real world shooting data.

 

This is no reason at all not to use tables etc as guides,but in principle we cannot depend on a very close agreement all the time,which is just what we often find-they don't agree!

 

Bramwell,D "Pressure Factors :temperature,powder,primers."