how to measure a 303 british 5-groove bore slug

[danyboy]

I have a 1943 long branch no4 MK1 303 british. I have slugged barrels before, usually 4-groove barrels. The only one I had trouble with measuring was the 7.5 x 73.5 schmidt rubin with a 3-groove barrel. Now I try slugging this 303 british and realize it's got 5 grooves. How does one measure a 5-groove bore slug with a caliper ?

[bruce drake]

http://castboolits.gunloads.com/show...he-Lee-Enfield
read the thread. great load of info on your question.

[mdi]

While it doesn't work for every one (I'm a life long machinist/mechanic) I have been able to measure very closely an odd rifled slug. I slowly spin the slug in a micrometer while slowly closing the spindle. "Feel" is very important here. Another method is to measure some shim material (very thin, .002"-.004") then wrap the material around the slug. Measure and deduct two times the thickness of the shim stock, and you should get a good measurement. There are v-blocks and jigs manufactured just for this measurement and Mr. Drake's link will surely include that tool...

[frnkeore]

This will give you a understanding on the land to groove ratio and how to measure them.

If everything is perfect, you can directly measure the red line. To varify it, measure the bore with a pin gage or by turning a pin that will barely fit. Then slug the barrel, measure how much higher the groove is, opposite the land. Multiply by two, then add the bore size.

That will give you the exact groove diameter.

Frank

[curator]

Better than a bore slug, make a "throat-slug" by upsetting soft lead plug in the section just forward of the cartridge mouth. The unrifled portion (leade) is the measurement you really need to select cast boolit diameter. Size your boolits no less than .005" under this measurement for best results. Another advantage of a throat slug is that it will show you how much longer your chamber is than the brass. Most military .303s benefit from slightly longer case length than 2.22".

[KCSO]

You call CDCO Machine co and order a V anvil Mike for $95.00 or mail me the bullet and for a small fee???

[W.R.Buchanan]

Use a gage pin of .303 to .306 to find the bore dia. Then using calipers measure the distance across the ID of the barrel.

If the bore is .303 and the measurement is .308 then you know the grooves are .005 deep and the groove dia will be .313.

That's all there is too it. I might add that most of the later #4 Mk1 guns with 5 groove barrels are going to be the dimensions mentioned above.

They are also left hand twist which was done on British and French rifles to counteract the Coriolis Force in the Northern Hemisphere.

American made guns with Right Hand Twists include Spin Drift Compensation built into the sights as far back as 1873.

Little tidbits I just found a few days ago. How they figured this all out 150 years ago is beyond me.

Randy

[danyboy]

I get it guys. I will try to find a pin or shim as thick as the groove depth and do the match after. Thanks to all.

[herbert buckland]

With cast just measure the mouth of fire formed cartridge from your rifle,I have found I get the best accuracy with cast by using a bullet as close to this measurement as possible,also saves working brass

[danyboy]

With cast just measure the mouth of fire formed cartridge from your rifle,I have found I get the best accuracy with cast by using a bullet as close to this measurement as possible,also saves working brass

Good idea. I'll do that. Thanks

[longbow]

As already mentioned, measure the throat or what Heb said, use the fired brass neck I.D. Basically you want the boolit as large as will fit the throat and still chamber.

Also, on that line, since most Lee Enfields seem to have largish chamber and bore dimensions you will find that your full length sizing die will size the necks down to suit the nominal 0.311"/0.312" bullet but if you are using 0.314" to 0.316" boolits then the neck will be expanded by the boolit and boolit swaged by the neck. Hard on brass and hard on boolits.

Best to get a Lee collet die and size to suit the boolit just getting enough neck tension to hold it in place. Otherwise you will be sizing down to 0.310" or so then expanding back to 0.314" or so which really works the brass.

Also, my die pushed the shoulder back pretty far too so with the collet die and neck sizing only that extra brass working is eliminated except for an occasional trip through the full length sizer if brass gets tight.

Good idea to regularly anneal neck to shoulder too, especially if you do full length size each time.

I bought a Lee collet die then made a mandrel to size down to 0.313" to suit my 0.315" sized boolits and realized much better accuracy and less brass working. I found out afterwards that the Lee collet die can be used without a mandrel so needn't have bothered making a custom mandrel. Oh well, it still works.

Longbow

[danyboy]

As already mentioned, measure the throat or what Heb said, use the fired brass neck I.D. Basically you want the boolit as large as will fit the throat and still chamber.

Also, on that line, since most Lee Enfields seem to have largish chamber and bore dimensions you will find that your full length sizing die will size the necks down to suit the nominal 0.311"/0.312" bullet but if you are using 0.314" to 0.316" boolits then the neck will be expanded by the boolit and boolit swaged by the neck. Hard on brass and hard on boolits.

Best to get a Lee collet die and size to suit the boolit just getting enough neck tension to hold it in place. Otherwise you will be sizing down to 0.310" or so then expanding back to 0.314" or so which really works the brass.

Also, my die pushed the shoulder back pretty far too so with the collet die and neck sizing only that extra brass working is eliminated except for an occasional trip through the full length sizer if brass gets tight.

Good idea to regularly anneal neck to shoulder too, especially if you do full length size each time.

I bought a Lee collet die then made a mandrel to size down to 0.313" to suit my 0.315" sized boolits and realized much better accuracy and less brass working. I found out afterwards that the Lee collet die can be used without a mandrel so needn't have bothered making a custom mandrel. Oh well, it still works.

Longbow

I see. Turned out that mybore size is .311 and groove size is .317. I will do a chamber cast as soon as I can and probably order a Lee decapping pin (mandrel ?) . Considering paper patching as I had good results with other calibers. I have a forster outside neck turner but will have to look for the wright size pilot. I will post my results.

[Ballistics in Scotland]

The class act for this job, but too expensive for the amateur, is a pin or plug gauge set:

http://www.ebay.co.uk/itm/Precision-...-/370960855206

I have a set of the smaller and cheaper ones for repairing antique pocket watches, and even at that price they are a bit of a white elephant in my tool collection. But here they are singly, at $3.64 each.

http://www.mcmaster.com/#gauge-pins/=yxjjmh

But you can surely get by with the butt end of twist drills or drill rod, and thin metal or paper shims. In the UK we can get metric ones in tenth millimeter increments, and the American N size is .302in. You can also pare down a larger-diameter, i.e. groove part of the slug with a knife or plane to get a pretty good measurement, and even edge-of-groove to edge-of-groove may be good enough for practical purposes.

Oversized groove diameters are mostly found in early No rifles. The worst situation, though one I don't know of anyone finding, would be one in which the chamber neck is too small to let you load a suitably oversized bullet, which in many cases makes these rifles work just fine. Usually you can do this, and bullets of .318 diameter for the early J-bore 8mm. Mausers, don't seem to take much harm from being sized down in moderation, using a simple die and a powerful reloading press. The Speer 275gr. .338 worked very well for me when sized to .330 for the 8x60R Portuguese.

[gpidaho]

I use the Lee collet Dies on several calibers and they work very well. On the 303 another good option is to size your brass in the Lee Loader (poor mans Wilson hand die) then expand the necks with a Lyman M-die or better yet a custom expander to fit the proper boolit. Most 303s have a lot of head space so don't size the brass back more than needed to chamber. Gp

[W.R.Buchanan]

I would measure that bore and groove dia. again. I doubt seriously that the bore dia. is .311 and with a groove dia. of .317 the bore is either seriously worn or more likely you might not have gotten good measurements.

You need to get the same measurement like 3 times in a row to insure you are getting repeatable measurements. Otherwise you are getting phony readings.

I am saying this as a highly skilled machinist who still gets phony readings even though I've been doing it for 35 years. This measurement is not that easy to get right everytime,,, I assure you. That's why you need repeat readings to verify the measurements.

First you need to use a gage pin to establish the actual bore diameter. Unless you do that or use a bore slug there is no other way to get an accurate reading on the bore dia. of an odd number grooved barrel. Then you can deduce the groove dia. by using calipers and referring to the diagram in post #4. Groove depths in rifling are typically .004-.005 and these numbers have been consistent in manufacturing since around 1890. If you are getting readings that are much different than what I am saying you probably are getting phony readings.

Springfield Rifles had barrels that were .3002 on the bore and .3082 on the grooves from 1912 on and didn't vary more than a couple of Tenths thruout their entire production of millions of rifles.

Long Branch guns were made in Canada and probably used the same machines as we used IE: Pratt and Whitney Rifling machines, to cut their barrels.

Barrel blanks are first Gun Drilled thru,,, Then they are reamed to size which establishes the bore dia. This type of reaming is done on an automated machine with flood coolant and will hold consistent diameters of the reamer for the life of the reamer. They don't vary more than a couple of tenths before they are resharpened, and they are closely monitored... Also the diameter gets smaller not bigger during that tools life.

Then the rifling machine takes cuts that are .0001 per pass until the desired depth of the grooves is met. Typically this is 40 passes per groove for rifling that is .004 deep. So on a 5 groove barrel with .005 deep grooves the machine would make 250 passes thru the barrel. It has been done this way for over 100 years. Before that it was done by hand pulling the cutter thru the barrel. The automatic machine is simply more consistent.

This process was explained to me by the rep for Krieger Barrels who still use the same machines to make match grade barrels that were used in WWII for rifling barrels.

I hope this clarifies this process more clearly for you.

Randy