Effect of hot barrel on POI

[30Three]

Gents,
I would like some advice regarding my No1MkIII*
Last week I used it for a competition at 200 metres. The first 5 practice shot's were on target, the next 10 shots were good (for me) scoring 79/100, but slightly lower than the practice group. However the second series of 10 had a noticeably lower impact position,(dropping into the white zone) while remaining a reasonable group size, it was only worth 64 points.
There was approximately 5 minutes between each series (5 then 10 then 10) and the shots fired within 3 minutes.
Could this vertical drop of POI be caused by the barrel heating up? If so, is there a remedy?
The rifle has a new barrel (old stock) with about 200 rounds through it. The wood is well fitted and the nose cap pad spring is new.
What do you guys think!
Thanks, Shaun

Information

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[Frederick303]

Shaun,

If you look at the 1931 armour notes or Jim Sweets little handbooks, the conventional bedding notes for the No 1 MKIII is covered in detail in both volumes. Either would provide a good guide for inspection of the rifles forend fitting.

Some years ago, when speaking to a Australian Armourer trained in the 1950s, he said that if the bedding was all correct but the rifle had a tendency to move the point of aim as the barrel heated up, there were 4 possible reasons:

1) The front trigger guard spacer was not the correct length, either too long or two short was an issue. He said the most likelihood was the spacer was too short and a slightly longer one could be tried., the basic reason was as the bearing under the barrel warmed up the tension on the barrel reinforce would cause the barrel to have too much up pressure under the reinforce and the point of aim would shift vertically (I think he said low but would have to check my notes).

He specifically said that in adjustment of the rifle that the tool used to adjust the length of the spacer for the front triggergaurd screw, that small variations in length would sometimes be required depending on the wood, though that might be due to the fact he was in Australian and the moisture content of the rifles might be drastically different on south Australia or Queensland. I think that is whey the forends were supposed to be soaked in linseed oil, to offset this.

2) The tension of the middle band was not correct. This was very unlikely, but could be if the two wood bearing surfaces on either side of the inner band were not at the correct level. I have heard competitors that adjusted the spring tension on this inner band, though myself I do not know exactly what the procedure was or how they troubleshot this.

3) The wood is relatively new and when it heats up there is a stiction condition created between the wood and barrel, that creates a different barrel movement profile shot to shot and as such the POI can move, though the likely effect is more of a scatter gun approach. The best fix for this is powdered graphite between the barrel and the bedding points where it bears.

4) A small number of barrels that were straightened in production tend to bend when they heat up and there not much can do with these.

The above was referenced in regards to rifles used in "Roberts" competitions, that is rapid fire matches where competitors start at the long line and run down form ~600 yards down to 200 yards, stopping to shoot a number of rounds at each yard line.

With ammunition that had the normal variations in velocity the No 1 MKIII will string shots vertically at 100 to 200 Meters (so called compensation). so if you are firing small groups of 3 shots, you could have the appearance POI shifts at the shorts, but it is really just the fact the rifle does tend to string vertically.

[30Three]

Thank you very much for your input Frederick. I will print your notes and inspect the rifle this weekend; check to see if anything has loosened off.
When I bought the rifle, it looked good, but was awful on the range. (trigger pull off, rear sight moving under recoil, worn muzzle, 6" groups at 50 metres!)

I've done a lot of work to get it to shoot well, however I had not really shot that many rounds in a short time frame, so had not seen this problem before.
The rifle is originally 1929 BSA, wood is replacement but of original manufacture. Over the last 2 years since getting this rifle; I have checked and adjusted the fit to ensure the barrel is in the middle and it has a snug fit on the recoil pads. I had to make brass shims to equalize the contact here.
There is no contact between the fore end and the barrel except under the knox form.
The barrel does contact the nose cap at about 11 and 1 o'clock. The up pressure pad spring is new.
Should the barrel touch the nose cap on the upper edge?

[Son]

A couple of quick questions.... Is it a standard or heavy barrel?
Do you use a sling when shooting?

[Gil Boyd]

Like Son, I was going to advise a few pointers such as using a stable rest and the best of the bunch is a sandbag, using the same point of aim each time.
I have always been one to blame the user first and not a proven rifle, and explore several techniques that can produce good grouping, especially if as you say you have a new barrel fitted correctly.

[Richard Hare]

30Three,

Too answer your Q, yes, the barrel should contact the muzzle cap at 10 & 2 position, and if you push a loaded round into the muzzle, you should be able to spring the barrel down away from the contact.
I am jot sure if this is also correct for a Heavy barrel, but is for the standard barrel.
I look forward to seeing you get this problem rectified. :-)

[30Three]

Thank you Gent's for your replies.
The barrel is standard type.
I was shooting from the prone position and using the standard sling around my left arm.
Occasionally I use a sand bag under the front for testing ammunition etc. But as the competitions are all shot without extra support, I prefer to practice as per competition rules.
Normally I use my No4 Mk1/2 for these competitions; but wanted to try the No1 as I had got it grouping well. However I had not shot the same quantity of shots in that time frame. Unfortunately I did not have the opportunity to photograph the targets; but the overall group size was similar, but just lower down the target. I am satisfied that my aim was regular. If I do pull a shot it tends to be lateral deviation as opposed to vertical.
I'll have to work out how to upload photos, so when I inspect the rifle I can post pic's of anything odd.
Cheers.

[Gil Boyd]

30Three,
From what you say, there appears to be very little to worry about as you describe the group has crept down the target but the MPI is still there, and the shots are all together.

IMHO that charateristic could be down to most probably the barrel as it heated up, as you mention numerous rounds in a strict competition timeframe. They do strange things from time to time and I am sure there are those more qualified than I to reply on the science of why that happens with certain metals used in barrels.
In short soldier on, but see what the rifle does next time out of a competition using a sandbag and replicate the rate of fire. If it creeps then its a charateristic of your barrel.

[30Three]

Thank you Gil.
If I can repeat symptoms then I can probably find the correct adjustment to counteract it; i.e. add a click or two after the first 10 round set.
This rifle has been a bit of a project for me since buying it . It's been lot's of little improvements to get it right. Fitting the fore end was probably the biggest improvement. Here are photos of before and after sorting out the fit of the recoil pads and general wood fit.

Attachment 72223Attachment 72224

These were shot from a sand bag at 50m, so you can see how bad it was!!

[Gil Boyd]

Sandbags the answer out of any Competition then, looking at that right hand target, and a tight sling positioned on the back of the arm when in a competition and tighten everything up, your grip, your hold and the butt back deep into the shoulder, that will really help!!