info on muzzle velocity ?

[terrywalsh]

My new 6mmTEW has given the following velocities;
1. 2827 7. 2996
2. 2971 8. 2978 stop for barrel cool
3. 2988 9. 3015
4. 2998 10. 3027
5. 2942 stop for barrel cool 11. 3011
6. 2980 12. 2994

Load was 45gr 2213sc. COAL 2.860 with 115gr DTAC.Rifle has fired 100rds during barrel break in and case forming. Rifle is a Barnard with 30" trueflite BR profile.
Does anyone recognise the pattern and why there is increasing velocity leading to a peak ?
terrywalsh

 

[Preacher]

Usually when a barrel is new it has a lot of little bullet grabbers in it that slow down the velocity, after the grabbers have been ironed out a little the velocity will increase to a point.... Also your laying down a layer of carbon that will slick up the barrel a little bit after each shot, and that will increase velocity....

 

[terrywalsh]

Thanks 'preacher' !

 

[Laurie]

Terry,

adding to what Preacher says, the low MV for your first shot is not unusual assuming you started with a clean barrel. The size of the reduction varies a lot depending on calibre and barrel make / condition - at 140 fps, yours is at the very top end of the usual range. This shows the potential effect on MV of even a modest amount of fouling.

Some rifles and barrels shoot more or less in the group from a clean barrel even at longer ranges, but it's common for the first shot to be of limited value as a sighting shot. If you have a setup like this and the type of competition doesn't allow a short fouling shot session ('Blow- Off' is the term we use in F-Class in the UK), or more than the usual two 'sighters' you either have to live with this or put two or three shots through the (clean) barrel a day or two beforehand to impart the requisite amount of fouling.

Some people get away with not cleaning rifles between matches, but while this often works for .308W in Target / Fullbore Rifle disciplines with their 2-MOA Bull ring diameter, I'd have my doubts about smaller calibres like your six and the advisability of doing this in precision competitions.

Laurie,
York, England

 

[terrywalsh]

Thanks Laurie ! I am switching to H1000 and magnum primers to get better case loading and see if there is any difference. My case has 60 gr H2O capacity(seems to be similar to 243AI, 6mm Crusader etc, so the slower powder should be better suited.
terrywalsh