How do I measure groove dia on a 5-groove .303 Brit barrel?

[strangwn]

I just acquired a .303 Brit No. 4 Mk1 with a 5-groove barrel and am trying to determine sizing for my Lee C312-185-1R #90371. I have not cast any yet, so do not yet know cast dia. I have pushed one slug through the barrel from muzzle to chamber. The slug will roll around with no tight spots between my caliper jaws set at .315, but tight spots start appearing at ~.3145. I plan to repeat the slugging process and use a mic, but my caliper is top quality. Assuming that my slugging is repeatable, all I will know is that groove dia is more than .3145. What is the trick to determining groove dia for a 5-groove? I have good mechanical skills, but am not a machinist.
Another concern I have is that my mould may not drop fat enough. The previous owner of the rifle and mould was sizing at .314. Does anybody know pros or cons about what I should expect from this particular boolit, as to drop dia and accuracy in my No. 4? My goal is to get this boolit(or some boolit) to decent accuracy at 1700 – 1800 fps, the C.E. Harris/Mattern “deer and 600 yard target load”, with help from this forum.

[madsenshooter]

In following a similar question on another board, a special mic is needed, I'll be interested in seeing the answers you get here as I have a 7 groove 35 caliber barrel I'd like to know the groove diameter of.

[twotoescharlie]

slug the barrel, wrap a very thin feeler guage around slug, mike it and subract the thickness of the feeler guage (doubled) from it.

TTC

[Larry Gibson]

Take a thin strip of shim stock material 1/2" wide by a 2 - 3" long of .003" thick. Other thicknesses can be used and even some of the really thin beer can material. Mic the thickness of what ever you are using to be sure of the thickness. Now make one wrap around the slug from your barrel and pinch it tight and hold it with the thumb and fore finger of one hand. With the other hand mic the diameter of the slug plus the shim wrap. Then simply subtract 2 times the thickness of the shim and that gives the diameter of the slug, i.e. the groove depth diameter of the barrel.

May not be accurate to to a .0000" but it is plenty accurate enough to .000" and for determining bullet size needs.

Larry Gibson

[madsenshooter]

Sheesh! This is similar to me wanting to know the bore diameter of an Obermeyer 5R barrel I have. After calling Boots, who told me to cut down a pilot until it fit then measure it, I was saying "Why didn't I think of that". Turned out I had an odd size drill bit that let me know it had a .233 bore. Thanks TTC and Larry

[DIRT Farmer]

I sluged the throat and sized for it.

[JeffinNZ]

I sluged the throat and sized for it.

Bravo!!!! Forget the barrel.

[Larry Gibson]

I sluged the throat and sized for it.

That's also what I do. In this case the question was on how to measure the slug from an un even land barrel. Many like to know the groove depth regardless of what they size for, I am one of them. Not sure it serves any great purpose to me other than simply knowing.

Larry Gibson

[JeffinNZ]

I sluged the throat and sized for it.

Better still, if you can chamber it, shoot it.

[DIRT Farmer]

Well if you can chamber it without a mallet

[10x]

A " V " block that is calibrated with a known diameter bullet can be used to measure barrels with odd numbered groves. If you have a mill these can be made very quickly.
Calibrate the block by measuring a bullet of a known diameter, subtract the diameter of the bullet from the measurement, then place the slug in the v block, measure it and then subtract the value for the block.

[dnepr]

A " V " block that is calibrated with a known diameter bullet can be used to measure barrels with odd numbered groves. If you have a mill these can be made very quickly.
Calibrate the block by measuring a bullet of a known diameter, subtract the diameter of the bullet from the measurement, then place the slug in the v block, measure it and then subtract the value for the block.

wow that makes so much sense , as a hobby machinist I should have figured that one out myself . as ussal I learned something new on this website

[10x]

wow that makes so much sense , as a hobby machinist I should have figured that one out myself . as ussal I learned smething new on this website

I've got several V blocks for measuring in my toolbox. Can't find one, make a new one...

[dnepr]

I sluged the throat and sized for it.

This idea is slowly sinking in to my thick skull but I am wondering about the limitations of this ,
I have done a chamber cast of my mosin nagant and slugged the barrel . it has throat of .317 groove and .311 bore (measured right in front of the tapered lead) which tapers to .314 groove .303 bore measured 2.00 " from the base of the neck , the barrel slugs at .311 groove. now .317 down to .311 sounds extreme to me but I will try it unless there is some reason not to. sorry about the thread hijack

[Pepe Ray]

My comments here are NOT to address the merits of slugging barrels or chamber throats.

For anyone bent on accuracy AND precision (you certainly understand that they are NOT the same), any V- block will only be reliable for one land/groove count.
The 5 grove bbl MUST be measured with a blok with a 108* V.
A 3 groove or 7 groove will each require a V of different degree cut. It is relatively simple to show or deduce those answers graphically but I have no interest in jumping into that mess at this time.

Hobbiests have successfully arrived at numbers that produced satisfactory results in deducing their boolet sizing needs by using any old V-groove, but it was only by good fortune. An accident!! A happy accident to be sure.

I'm just trying to keep the record straight. = = Good Luck
Pepe Ray

[pipehand]

This post prompted me to slug my No.4Mk.2. Haven't checked the throat yet, but using some LBT push thru slugs and a strip of .005" beer can wrapped around the slug I keep coming up with .308 after deducting .010" for the double thickness of the aluminum. I didn't believe it at first and mic'ed a new slug to be sure it wasn't .308 to start. Nope, .316+" was the unused dimension. Came up at .308 the second time I slugged it.
That's not supposed to happen, even with a 1955 Fazakerly in pristine condition (which this rifle is). I have a few boolits cast fron an NEI 30 cal paper patch mould that measure at .301". It is a press fit in the muzzle, leading me to believe the bore size is .300". Standard American 30 Cal!
This rifle has been sort of a safe queen. I did put some Greek HXP through it around 16 years ago, a few cast loads with 10 grains of Unique, and it shot very well with some .314" pistol bullets and a popgun charge of Unique last weekend. Its good to know my boolet choices aren't going to be limited by a really large groove diameter.
This is a .303 but it looks like someone started up tooling for the 7.62 NATO barrels a few years early. Anyone else come across one like this?