View Single Post
Old 2nd November 2015, 03:15 PM
RobF's Avatar
RobF RobF is online now
My Empire of Dirt
Join Date: Mar 2010
Member of: Southampton Buccaneers, Parkstone, South Dorset
Location: Poole, Dorset
Posts: 10,171
Default Cant and scope alignment...

Ok, so working through a list of excuses of why I didn't win the worlds last year (taking Mr Privett to the airport is one) is a something that kept bugging me with the TX's zero and something I didn't have time to sort.

On two ranges, in very light to almost no wind, despite getting good groups I was having a zero issue. This was that what felt a good zero t at 50-55 yds was giving me about 4-5mm of windage left at 10yds, and if i dialled that out I was about 20mm out at 55. Indoors bouncing between 10 and 25yds I thought I saw something, but I was also struggling to get a good natural level hold... so i put it down to that. Come Lithuania and I spent 3 days shooting for wind on long shots I couldn't see... which isn't really the way to do things...

I wanted to test it indoors before I went, but didn't get the chance.

Anyway, whilst staring at the gun I realised the only level I had worked with on the gun was on the mounts. On the TX there's nothing level. As much as I trust AJP's rails, it's still not ideal.

So we threw one on the lathe (a dyno donkey, not my race rig). Got the barrel true to the lathe and put a dial gauge on the action. That (as expected) wasn't spot on, but what we did find that although the dovetails were straight (and we'd checked them to be level as well), back to front we have a deviation of 0.4mm away from the barrel over the length of the dovetail. After getting that straight we put a scope on. What we found was that was out by about 2mm left. Probably due to the mounts.

Now here's the ponder...

If we level the gun and we level the rifle, there is no guarantee that the scope's vertical axis sits over the axis of the rifle. If the mounts don't come central, it might be left or right by a mm's.

Doing some quick sums, a 2mm shift with a 50mm scope height is about 2.5 degrees of deviation from the axis of the bore.

Now is that the same thing as cant? Because just 10 degrees of cant at 50yds seems to suggest 1.3" at 50 yds... according to this website...

So I'm wondering if there's a maths boffin out there that can;

a) work out if offset of a scope is actually the same as cant, or there's another sort of error

b) if it's possible to calculate the effect

c) if it's all voodoo

In the meantime, we'll me making sure my rig is straight, level and my scope is central. That means that my dovetails may not be so that the scope can be rotated over the barrel so it is central, then we need to lock on a level and that's to be the new level. I hopefully wont need to change my mounts so i wont need to do it again.

Then i'll test it indoors at 50m and possibly 100m
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep
Reply With Quote
Connect With Facebook to "Like" This Thread