Velocity inconsistent

[smokeumm]

I'm testing the berger hunting vld 168 gr bullets in my 7mm stw. The speeds I'm getting vary from 3,240 to 3,285. This load shots great but I'm worried about the inconsonant velocities. I'm new to chronographing loads, so is having a load plus or minus 45 fps reasonable??? If not, what should I expect the variance to be?

 

[MTBULLET]

need a little more info to help come up with answer, how far from muzzle is crono? how many shots to average? what crono? battery new in crono? what is vertical spread on target and what range to target?

 

[smokeumm]

The chrono is 10ft from muzzle. I've shot ruffly 10 shots. The battery is a few months old, but the chrono has maybe been used 5 times since I bought it. It is a Competition Electronics ProChrono Digital Chronograph. The range is 100 yards and my grouping is good, maybe 1/2 of an inch,, no more. The vertical spread is around 3/8 of an inch.

If everything is dialed in with a load, what is a reasonable variance in speed?

 

[MTBULLET]

move crono to 15' from muzzle, and make sure it's level. also, use sky screens all the time.

 

[cowboy]

move crono to 15' from muzzle, and make sure it's level. also, use sky screens all the time.

AGREE with the above. At 10' from the muzzle you are possible getting some false readings due to muzzle blast. In fact I put mine at 20'. Keep in mind that at that distance you need to go back into a balllistic program to get "true" muzzle velocity which I would guess will be about 10-15 fps faster than your chrony is reading.

 

[Lefty7mmstw]

The news is out... velocity will vary a bit from shot to shot.....

A bit of variance is normal. You should try to stay away from 100 fps extreme deviation by a long shot, but a 20 fps or so x like you are talking about isn't that bad. Like the others have said, get the chrono back a few feet(15-20) and level, use diffusers or sky screens, and then you'll know what your real variation is.

Btw, 3263 fps average is 100 fps or more hot for an stw with a 168, if you drop your charge to a more reasonable level, your x's may come down some. At the least, you'll be saving bore and action life and getting more reloads out of your brass. General rule with an stw is 3400 with 140's, 3300 with 150's, 3150-3200 with 160's, and 3000 with 175's. Your 168 will be at 3100 fps at normal top pressure with a 24-26" barrel. I've worked with a bunch of these rifles, and none has been uniform well over pressure.

 

[smokeumm]

Thanks for all your help and ideas? I will try 20 feet with a screen!

I thought the speed I'm getting was quite high myself. This is a custom rifle made by McWhorter. The load data supplied by McWhorter is 83 grs Vihtavouri N570 powder, with 168 grs vld berger. The last thing I want to do is to damage this gun. If this load is unsafe for the rifle I will for sure tame it down???

This gun shoots excelent, but I have not tested it for long range shoting. I'm in the process of learning how it shots first.

 

[Broz]

move crono to 15' from muzzle, and make sure it's level. also, use sky screens all the time.

A big +1 for sure!!

Jeff

 

[elkaholic]

Maybe I missed it but I didn't see anything concerning case prep? If you are not already doing these things, they will tighten up your spread. Sort your cases to no more than 2 grains variation. Make sure the necks are turned consistently to the same thickness and trim to exact length. ANNEAL THE CASE NECKS! This will help keep the tension more uniform case to case and thereby help equalize velocity. To prove the point, number 6 brass from 1-6 and then carefully load and chronograph each. Take the three most consistent and separate from the three most extreme and fire again checking the velocity. I am pretty sure you will find that the results (case to case) will remain consistent and the three that started out the closest will remain that way.......Rich

 

[Fiftydriver]

I do get a kick out of guys bragging about their rifles that get single digit extreme spreads. Thats great if they are actually doing it and there are some guys on here that are reporting such results that I believe 100% because I know them personally but on average, this is not attainable or even needed.

Generally speaking the lowest extreme spreads will come from larger caliber chamberings, 338s and 375s are famous for low ES velocity numbers. As the bore size gets smaller, especially if case capacity stays the same, ES usually will increase on average. For example, its pretty easy to get a 338 Edge to produce ES well under 20 fps and often into the low teens and even single digits for a given 10 shot string.

A 300 RUM will generally easily get into the sub 30 fps ES range, often times into the 20 fps range but its rare to see a 300 RUM get much tighter then that.

A 7mm RUM will generally break well under 50 fps ES and often times into sub 40 fps spreads but its rare to see one get much under 30 fps for a 10 shot string.

Why is this...... Well, my theory is that the slower burning powders simply are not as good at producing very consistant velocities as the quicker burning powders are. That is not to say they are inconsistant, just not quite as consistant as the bigger bores. Also, I believe that in the smaller bores, minute variations in temp, Bar pressure, bore temp, bullet jacket thickness, bullet weight, baring surface length and several other areas will make a larger effect on final muzzle velocity on the smaller bullets then on the larger caliber bullets.

An example, if you have a group of 175 gr 7mm bullets that vary by 2 grains over 20 bullets, thats a 1,14% varitation in weight.

If you take another 20 bullets in say a 300 gr 338 version that have the same 2 grain variation, thats a 0.66 % variation.

Now its not uncommon to see weight variation in bullets, Baring surface lengths can vary greatly as well and that makes a much great difference in the smaller caliber chamberings.

Simply put, if you get a load that will produce good consistant, ROUND shaped groups at 100 yards and has an ES of less then 50 fps. Test that load at long range before you start chasing that magical ULTALOW Extreme Spread. A 50 fps extreme spread will make much more difference on paper or on your ballistic program then it will in reality when actually putting bullets in the air. Much over 50 fps and yes I would say try something else.

If your under 50 fps acceptable if the load is consistant in the accuracy department. Under 40 fps is good, under 30 fps is great, under 20 fps extremely good and anything under that, well, going to ruffle some feathers here but YOUR LUCKY!!!

I do not get to hung up on velocity spreads. In fact I seldom shoot a 10 shot string to get a meaningful ES. Shooting at long range will instantly tell you if your velocity spreads are to high as you will see noticabl vertical variation in your shot impacts.

Have seen many rifle/load combos that would produce easy sub 3/4 moa groups at 1/2 mile that had extreme spreads of between 30 and 50 fps. I even own a 22-250 AI that will put the 55 gr Ballistic tip into sub 2" groups at 500 yards even though it has a legit 90 to 100 fps ES for 10 shot strings....... GO figure....

Don't get to hung up on extreme spreads, thats a good way to waste more of the accuracy life of your barrel!!! Test at long range, use a chrono to get a base idea of your muzzle velocity and then start testing your drop charts at long range. In the end, you will need to correct for your MV and many other factors to get your drop chart to match up perfectly with your actual trajectory so in the end, it means very little to putting your bullet on the mark that first shot.

 

[Fiftydriver]

Couple more things, if you have a bare muzzle, yes, get at least 15 feet off the chrono, if your using a good muzzle brake, let me rephrase that, if your using a good partition style muzzle brake, you can shoot at 10 feet or even closer with good accurate readings because the muzzle brake will redirect the muzzle blast and keep it away from the chrono.

Keeping the chrono level will give you the most accurate velocity readings, it will do nothing to change the consistancy of your velocity. If you chrono is not level, the distance between the two optical eyes will be shorter, therefore it will take less time for the bullet to cover that distance so it will tell you that your velocities are faster then they actually are. BUT it will not give you incorrect ES numbers.

A fresh battery can be a good idea also.

Shooting light can be critical with a chrono. I prefer a day with light cloud cover that still allows good light and generally speaking, shooting during the middle part of the day has been most accurate for me. Shooting early in the morning or late in the evening can produce poor shooting light and really cause strange Chrono readings. Very intense sunlight can also cause poor chrono readings as can heavy shadows.

One trick I learned a long time ago was to black out the ogive on your bullets that your chrono testing. This is generally not needed in the calibers we geneally use for long range hunting but it can help if you have a finicky chrono. I use a black sharpy and simply PAINT the exposed portion of the bullet.

I learned this trick a long time ago when trying to chrono 17 cal rounds. When velocity got over 4000 fps it was nearly impossible for the chrono to SEE these bullets. Blackened the tips and all was well.

Also happened when I was using some of the 57 gr Calhoon 22 cal bullets with their Slick Silver coating, the chrono did not like this shiny silver coating, again, painting black solved that problem.

As mentioned, generally you do not need to do this with larger calibers but if you are questioning your readings, it can help.

As far a case prep, here i what I recommend for a long range HUNTING rifle:

Required
-Debur flashhole
-Full length size virgin brass
-Trim all case mouths square and to the same length
-Evenly debur inside AND outside of the case mouths
-Seat primers with even pressure
-Measure powder charges to +/- 0.1 grain
-Seat bullets to within +/- 0.001" in length measuring Ogive to land clearance, not OAL.

Additional things you can do if you want to get into it. In my opinion, these next steps WILL improve your consistancy but for shooting in field conditions they will not offer the returns as high as the above items. The one exception would be for EXTREME range shooting, that being over 1K yards.
-Sort cases by case volume (not by case weight)
-Sort bullets by weight
-Sort bullets by baring surface length
-Neck turning (take very light passes to even up neck thickness)
-Square primer pockets and cut to uniform depth

In my testing the above list will get you the most bang for your buck for a long range hunting rifle, the second list is for those that like bench work or will just sleep much better then if they did not do these steps. I sleep just fine not doing them though.