Esteemed contributors to the CB forum-

I would like to confer with you regarding some observations today while doing load development.

I would like to know whether you have noted a greater sensitivity in vertical dispersion for CB's vs. "fully gas checked" bullets as a function of velocity variation.

In working to push up velocity for a hunting load for my .348 Win., with a variety of powders tried I have observed a significant difference in point of impact that appear to directly correlate to the velocity. Extremely sensitive is on the order of 9" for 250 fps delta - AT 50 yds.! (for your reference - 1950fps vs. 2200fps with multiple stops between) Small variations on the order of 50 - 60 fps show up as dispersion on the order of 1.5" to 2" at this range.

Questions:
1) Are cast bullets more sensitive to velocity variation than their jacketed counterparts?
2) If yes - it seems to call for loads that are much more consistant with lower ES in order to minimize the variations. Is this correct?
3) Aside from powder choice & charge weight - are there other variables which can be controlled to minimize this effect.
4) If something else is at work here that any of you has experience with - Please Share!

This has honestly come as quite a surprise to me. I suppose I had not noted it before because my load development was not done at the range where I incrementally changed the powder charge as I am now. The 9" POI change at 50 yds is a real shock - but it has been verified with 4 different powders. In fact - consistancy wise - I could pretty much call the velocity by looking at the shot landed on paper.

Thanks for your assistance.

Best regards-

Sky C.

Sky C., There may be a correlation bet. velocity & vertical dispersion, but why would it be greater for CB's than jacketed bullets? I've experienced vertical stringing with CB's too, but never that large. Some possible explanations may be poor bullet to leade fit; wrong sizing diameter; too much or too little bullet lube; bullet isn't stabilizing @ 50yd. (Have you found this @ 100yd. too?), which suggests a problem with twist rate; bedding/bbl. heating "issues;" shooting technique; and possibly too small a sample. One further question: Were your standard deviations greater than say 50fps? If so, could the powder you were using be a contributing factor? Just some food for thought. ...Maven

I'm assuming this is a lever gun. The various bands/dovetails/etc. tend to cause the barrel to whip up/down in response to longitudinal stress. I have seen this kind of variation with a brand new Marlin 30-30 shooting full-house loads with 130gr and 150gr bullets.

I'd be willing to bet the cause of the change of impact has a LOT to do with the amount of time traveling down the bore. Hence, the effect of recoil on point of impact.

Question: did the point of impact go up or down with increased velocity?

That much spread screams "bedding problem" at me. Yeah, I'd count problems with a tube mag and barrel band as a bedding problem.

Could be that the CB load is exiting the muzzle near a "loop" of vertical vibration when the barrel tip is moving fast. A small change in exit time could make a big difference in the bullet's path. The less sensitive FLGC bullets may be fortuitously reaching the muzzle near a vibrational node when the muzzle's not moving much.

Sky C.

I ran some ballistic calculations to determine the bullet drop at 50 yards for a bullet with a B.C. of 0.300 launched at 2200 fps and also at 1950 fps. The drop for the 2200 fps was 0.9 inches and the drop for the 1950 fps was 1.2 inches.

In case the lower velocity was causing bullet instability, thereby lowering the B.C., I used a BC of 0.100 and ran the 1950 fps again. The calculated drop a 50 yards was 1.3 inches.

I would appear that the vertical dispersion you are experencing is not directly related to a difference of the bullet's gravity drop as influenced by the small difference in time of flight that is caused by the velocity differences.

Gentlemen-

Thank you for your review of the situation and comments thus far. To answer some of the questions posed...

1) Lever gun - Browning M-71
2) Bullet appears to be stable. Clean round holes printing at 50 & 100 yds. Greenhill formula says there should be no problem for this bullet length (1.010") with the rifling twist rate (1:12)
3) Shooting technique - Shot off of solid bench, forearm held in hand supported by sandbag, rifle held tight into shoulder. In general - my shooting with other large caliber rounds has not been a problem - have not had problems shooting on the order of 1" or slightly less at 50 yds using receiver peep & bead front sight with good loads. Even so - bench technique is open for review.
4) Vertical POI seemed to be consistant in relation to the velocity - just very sensitive.
5) For load development - I started with a low charge and was incrementally increasing, working velocity up while looking for a "charge range" where the POI might have been relatively insensitive. I didn't find it. Each change in velocity move the POI vertically. The faster the velocity - the higher it printed above aiming point.
6) SD not record because I was only shooting three shots. ES for the loads tried was not too good - say on the order of 100 fps. Coupled with the POI/velocity sensitivity - grouping was poor and I went no further.

Bedding issue seems like a good place to look. - Will pull fore end (and perhaps magazine tube) and try shooting to see if this is where the problem lies. Thanks to all for the suggestions.

Best regards-

Sky C.

Have you tested jacketed bullets at the same velocity to see if there was any relation to fps?

Hello Ric-

Did some initial testing a while back with jasketed. POI roughly correlated to the CB's for same velocity. I haven't run anything jacketed down the bore on this gun in quite a while intending it to be CB's exclusively.

Best regards-

Sky

In normal instances the faster the velocity the lower the impact of the bullet because the less barrel time the bullet has the less the barrel elevates thus the bullet leaves at a lower angle.
I know it can have the opposite effect, but not usually. I'm wondering if the rifle is bouncing up from recoil before the bullet has departed the barrel as the velocity and recoil increase. Maybe try a test of not resting the rifle nor your hand on the sandbag...that is more or less off hand from the bench. See what that turns up.
Joe

As you state "bench technique is open for review".
I have read, when benchresting lever and break-open guns, that better results were obtained by putting the front rest under the action, rather than the forearm. Just something else you might try.>>>>>>>>>>>>>>>>Bug.

Tom you might remember the bullet pics I've sent over, some of the LBT and 308329 Lyman spitzer. Putting together an HBC design on another thread here-- the Q comes to mind, do lube grooves affect BC in your calculations/opinion/experience?

I believe it's minimal if the bullet isn't shedding large amounts of harder lube going downrange and if those grooves aren't near the ogive transition from bore to groove diameter.

Starmetal & Bug-

I've been turning the situation over in my head a bit & the bench technique issue has risen to the top. I too was thinking that perhaps I'm seeing recoil related POI change. Please advise thoughts about the following:

Trying to isolate velocity related POI change vs. recoil related...

Thinking to load 3 charge weights - say a grain apart ea. With current powders - that should provide something on the order of 120 - 150 fps difference velocity between low & high.

Using above loads - Shoot with same technique as last time - check for (verify) POI effect.

Shoot prone or sitting - compare POI effect.

My thinking is that if it is recoil induced POI shift based on bench technique - I should see a difference in POI spread between the groups. If not - concentrate on other areas.

Will report after getting out to the range again - hopefully within the next couple of weeks.

Best regards-

Sky