Ladder testing

[mparsons911]

Hello All,
Rookie here, neading a little explanation. I was helping a friend develop a load for his rifle. We found max load easily enough. Then we loaded backwards from max load going down 1% in powder for each load for about 9 loads. We started shooting and as the loads went up in powder using the same point of aim, the shots hit lower lower on the paper.
Somebody please explain this to me.

 

[Kenny474]

Faster bullets leave the muzzle before the rifle has a chance to recoil fully, so slower bullets will still be in the barrel as it rises from recoil. Confusing until someone explains it.

 

[TonyR]

Jason Baney's method described in this link has worked well for me and should answer your questions:
http://www.6mmbr.com/laddertest.html

 

[mparsons911]

I thought the bullet being in the barrel longer during muzzle rise explained why lower charges resulted in higher bullet holes at 200 yds (Which is what my test shots did, rise for less powder).
But the ladder test shown in a response and the ladder test I was following clearly show lower charge equals lower bullet holes. I'm still a ttle confused.

 

[Laurie]

It can also depend on how the barrelled action vibrates. In extreme cases you get what's called negative or positive compensation. The old British Lee-Enfield action was famous for positive compensation which allowed it to shoot better at long ranges than short. The action and barrel flex under firing pressure, and the exact position of the muzzle at the time of bullet departure varies according to how long the bullet takes to travel along its length ('barrel time'). In the case of the Lee, the muzzle moved upwards during this process, so a high velocity round saw the bullet leave at a slightly lower elevation than the norm, while a 'slow' bullet left it fractionally later thereby getting marginally higher elevation, the barrel having flexed upwards a little more. In an age where the factory .303" milspec ammo used in British rifle competition had large MV variations, this effect gave much improved elevation consistency from around 600yd range and out. A 'fast' bullet was in effect aimed slightly low and a 'slow' bullet aimed a little high, and with luck they all hit in the same place.

Two lug Mauser type actions with the lugs at the 6 and 12 o'clock positions when locked including the Remy 700 and Winchester 70 display 'negative compensation' behaviour, so the muzzle is fractionally lower for high charge and velocity cartridges making the bullet strike also lower with increased pressure and MV. Much action design work in recent designs has been to either get rid of this phenomenon, or to obtain positive compensation. The stiffer the action, the less flexing and the less trouble, so single shot actions with a solid bottom and small ejection port plus thick walls behave better than an out of the box Remy even though they retain the basic design.That's also why the benchresters sleeved Mauser and Remy actions in pre BAT, Stolle, Lawton etc actions and when these weaker sporting rifle type actions were widely used in the discipline.

Alternatively, many multi-lug target action designs such as the Barnard and RPA quad numbers are designed to give mild positive compensation, as well as being built very strong and stiff, and having very good locking lug to receiver fit-ups.

Laurie,
York, England