Vertical stringing

[txjm]

Ok, I finally got my version of the 7.62x40 20" barrel to function. It even locks after the last round. Clipped a coil, lightened the buffer and opened the port to .125".
Now, what is wrong when an AR vertically strings groups? I am shooting a 20"+ take off remington barrel that I cut threads on, a high rise upper and a free float handguard. I am shooting on bags. I am shooting less than .75" groups with my 223 upper. I am getting a perfectly vertical group with this one. Maybe .75" x 2" maybe .75 x 2.5". I have gotten some really awesome 3 shot groups but if I keep shooting the groups alway end up stringing vertically.
I seem to remember somewhere I read that it may have something to do with the torque on the barrel nut being too loose or too tight??? Any suggestions??
Chip

[Spook]

assuming your powder charges are precisely equal... use slower powder or shorten the barrel

[txjm]

shorten the barrel

Youre killing me. I love the velocity Im getting and I just got it to function. I know the shorter stiffer barrels are better in some cases but can you think of anything else? Is it harmonics? I don't see being able to change powder. Im pretty limited on bullets. Muzzel brake / weight might change harmonics??
Chip

[Alleycat]

A slower powder will not necessarily drop velocity. Have you checked the velocity for consistency?

[txjm]

A slower powder will not necessarily drop velocity. Have you checked the velocity for consistency?

I do need to get my chrono back out. The rifle now functions consistently to check that kinda thing. I haven't heard much in the way of effective powders other than 1680 in this round. I did try 4198 and didn't get the velocities. I do fireform with h110 but it wont cycle the rifle reliably. I'll get the chrono out and check SD.
Chip

[kurtz]

glad to hear you got your 20" running.....I've had pretty good luck with 1680 and H4227 in my 20" 40 rifle length sytem....usually keep the velocity between 2,500 and 2,600 fps with 125s.....

most of the stuff I've messed with issues of vertical stringing was traced back to shot to shot velocity variance rather than something mechanical, but it's a good idea to check the system as a whole....

velocity variance can be caused by many things: variances in cartridge length can cause pressure swings in land jump and burn rate (especially if using compressed loads), neck tension, charge repeatability, chamber heat soak from shot to shot, barrel / bore heat changes, chamber and bore fouling.....the more you shoot there is an incrimentation of heat build up in the upper half of the mag (sometimes to the point that the copper will turn rose colored, can cause powder burn swings)....several of the guys that I know that shoot DI ARs for groups single load at a set cadence to minimize mag and chamber soak temp issues....

[txjm]

....the more you shoot there is an incrimentation of heat build up in the upper half of the mag (sometimes to the point that the copper will turn rose colored, can cause powder burn swings)....several of the guys that I know that shoot DI ARs for groups single load at a set cadence to minimize mag and chamber soak temp issues....


I have had a quite a few that the first 2 were touching then the next would be high but nothing I can count on. I will get the chrono back out and see if I am getting a velocity diff that I can correlate.

Thanks kurtz
The 20 is awesome now that it is fully functional. Pigs this weekend.
Chip

[Medic650]

Where can I find more info on the 7.62x40? I hadn't heard of ot except on this forum.

[Fudmottin]

Sounds like a 223 case that isn't cut down as much as for 7.62x34, AKA 300-221.

As for the solution, I think the usual suspects have been mentioned.

[txjm]

"Where can I find more info on the 7.62x40? I hadn't heard of ot except on this forum."
I believe Kurtz is the man on the 7.62x40. I think he has a really long string on ar15, in the Variants section I think.
I think you can get any info that you would want from Kurtz. I think I saw info on barrels and dies also.
Mine is nothing more than running a 300w reamer in deep enough to seat a 125 comfortably at mag length. I was chambering for the 300w and noticed that the reamer was on the small side and just decided to run it in deeper to see how it would work. Ive had no issues. The brass is holding up well and it feeds good. I think my shoulder is farther back than kurtz's 40 and he has a little more case capacity. I didn't have much to lose, I was working with a barrel someone gave me left over from a remington donor action. I may still go back and see if I can get ream this chamber again with a 7.62x40 reamer.
If I was to do it again I would go with a 762x40 because of the availability of dies. I am messing around with multiple dies to resize.
I like my 300w but the 762x40 has some advantages on dedicated supersonic uppers. It has become my pig rifle.

Link to 762x40 string
http://www.ar15.com/forums/topic.html?b=3&f=121&t=382761

[pud-knocker]

with my stuff if the vertical stringing shots hit progressively higher it usually shows up on the chrony as higher fps, I always just chalked it up to everything heating up.

I really like my 40s, have two of them: a 20" mid-length gas system and a 10.5" pistol.

Another thread: http://www.quarterbore.net/forums/showthread.php?t=1827

[Rikky Lee]

Hope you went to a faster powder. The faster powder improves the consistency of the ignition and therefore smooths out velocity spikes.

[LouBoyd]

Diagnose your situation.
1. Record the velocity of each shot along with the position of each shot on the target.
2. Using a ballistic computer check that the vertical stringing correlates with the variation in muzzle velocity. You don't want to chase a loose scope
by adjusting loads.
3. How good is your velocity consistancy? Calculate it in terms of standard deviation and extreme spread.
4. Vertical stringing increases with the SQUARE of the distance for a given velocity variation. What range are you shooting at?
5. Are your expectations for vertical stringing realistic?

for any bullet without atmospheric drag at 1000 fps the time of flight and drop are:
yards sec feet
100 0.3 1.44
200 0.6 5.76
300 0.9 12.96
400 1.2 23.04
500 1.5 36.00
600 1.8 51.84

Real bullets of course slow down with distance so the time of flight and drops are always greater than the chart above. Very high BC subsonics like the 50 Whisper with Barnes solids are close to the above chart. Pistol cartridge subsonics are much worse. Calcuate the expected drops for the bullets you're using.

A 1% change in velocity gives close to a 1% change in the total drop.
For 1000 fps a 1% change is 10 feet/second.

1% variation high BC ammo will string about 0.17 inch at 100 yards. (one hole groups). It will string a lttle over 4.3 inches at 600 yards. That's still good for hunting with careful range measurement.

You shouild be able to get that velocity spread with careful handloads. You should expect some effect in that order from a cold to warm barrel. With careful powder selection that can be improved a little. It's very unlikely you can achieve 0.1% or 1 fps variation even with the best benchrest loading techiques. If you're getting greater than 20 fps variation you can probably do better.

Use a program like Quickload to check:
what is you % case fill? Around 95% is most desirable. Use a less dense powder to raise this number. Under 80% will hurt velocity consistancy.

What is the peak pressure? Around 90% of SAAMI max is ideal for safety and long gun life. Under 80% will hurt velocity consistancy. Faster powder raises this number. There are jumps and gaps in avaiable powder density and burn rate. Sometimes the only solution is to use a different cartridge.
That should not be a problem with a 300 Whisper and available bullets.
Fortunately the 300 Whisper is a high pressure cartridge. Use it as such with fast bulky powders for accuracy. That may not be best if you're trying to also cycle a semi-auto.

Do your bullets seal the bore before firing? That's desirable particularly for subsoniics to prevent blowby. Blowby can cause huge velocity variation. Losing 1 grain of propellant gas before the bullet engraves makes a lot of difference if you only have 4 grains to work with.

Are your bullets the right diameter? If the bore is slightly oversize or the bullet slightly undersize there will be blowby and velocity consistance will be bad. Too large of bullets will give high engraving force and high barrel friction which also hurts velocity consistancy.
Here's a speed video clip showing blowby from a slightly undersized subsonic bullet. Note the the smoke >before< the bullet reaches the muzzle. http://www.kurzzeit.com/eng/startseite.htm

Is bore friction variable? Bore friction doesn't necessarily have to be low, but it must be repeatable. Velocity will be more uniform if the ratio of chamber pressure to engraving force and bore friction is high. Lubricants
will lower bore friction but they don't necessarily make it more uniform.
That requires getting the right lubricant and application.

Are you using a long barrel? Most subsonics need only 8" to 12" barrels for best accuracy. 16" may be needed - for legal reasons. Longer than that is just throwing away accuracy. Again, the ratio of chamber pressure to bore friction needs to be high or the variabilty of the bore friction from shot to shot can ruin velocity consistancy.