No 4 mk2, warm barrel = change of POI, WTF?

[LeftFootOfDoom]

Hello all:

I am in the process of ordering a new front sight for my No4 (current is .045)and went to the range to confirm what size I needed. I discoverd something that is puzzling and annoying me.

At the military shoot several wks ago at the Islington range I noticed that POI was about 7-8 inches below the POA. The weather was extremely hot that day, and the rifle was in the hot trunk for an hour long drive to the range.

Today at silverdale went to confirm the low POI. Weather was cool today, and with a cold barrel the no4 is shooting spot on POA. AFTER about 20 rounds the POI drastically changes to 7-8 inches below POA again. GRRRRR.

1) Does anyone know what would cause this drastic change?

2) Is there any way for an untrained individual with minimal tools to fix this problem?

3) If there is a fix, where abouts will the new POI be? Will it be consistent with the current cold barrel POI, the warm barrel POI or somewhere in between? So I can figure out what size front sight to get.

Thanks in advance

edit to add:

-all shooting was done at 100m
-same ammo for both shoots, igman softpoints
-full wood no 4 mk2
-not a wandering POI, cold barrel had one definite POI, warm barrel had different POI, both consistent.

This makes Mr Bigglesworth angry, and when Mr Bigglesworth gets angry, people die

 

[BadgerDog]

How about a reader's digest version......

Temperature - affects the burn speed of the propellant, the air density (and therefore the trajectory) as well as the expansion or contraction of the rifle.

Humidity - affects the air density (and therefore the trajectory), the burn rate of the powder as well as having an effect on the moisture content of the stock in a rifle with a wooden stock, leading to inconsistency of the rifle components.

~Angel~ finds that groupings of her military "full wood" Enfields vary considerably. She also found that her No.4(T) sniper rifles were fairly accurate and predictable at 400-500 meters for the first couple of shots, then the groups began to open up and rise as the barrels heated up.

Regards,
Badger

 

[maynard]

Sound like a bedding issue. Some of the older #4 shooters SR(A) and SR(B)tell me they used a center bedding method. This is were the barrel was bedded for about 2 inches at about the midle of the barrel.

 

[cosmic]

You dont mention the yardage ... Assuming 100y, and the same ammo, you should consider the bedding to be suspect. Is the barrel free to move at the fore-end? I cant recall the prescription - something like 2-3 lbs force required to lift the barrel off the bottom of the channel, no other contact up to the Knox form....