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I’ve noted a few folks besides me who’ve scratched their heads about military surplus Mausers in 7x57 with groove diameters of .287, quite a bit more generous than the US standard .284/.285. Most explanations I’d read never made a lot of sense to me, so I kept digging and eventually, I found the following information in an article in Rifle magazine, “The 7x57 Mauser” by Ludwig Olson, from 1991. “Mauser Company specifications for 7x57 rifling were: land diameter, 7mm to 7.05mm (.2756 inch to .2776 inch); groove diameter, 7.25mm to 7.3mm (.2854 inch to .2874 inch); groove width, 3.9mm to 4.1mm (.1535 inch to .1614 inch); four grooves of concentric type with right-hand uniform twist, one turn in 220mm (8.66 inches); maximum bullet diameter, 7.25mm (.2854 inch). Metric figures showing land diameter, groove diameter and twist rate, along with proof marks, were stamped on the bottom of the barrel near the breech end.” So groove diameters of .287 were within Mauser factory tolerances. Based on measurements I’ve taken from various 98s in 7x57, I suspect that .287 is more common than .285, but that’s just a SWAG based on a small sample. The two VZ 24s I have in 7x57 both have .287 groove diameter (Brazilian “Sao Paulo Constitutional Revolutionary” rifles, rerouted from shipment to China for nonpayment, intercepted by the Brazilian government and repurposed as police carbines - are known as the VZ 24 JC) but I have no way of knowing if that dimension was standard for Brno produced 98s or not. I do know that some Brazilian military ammo was ordered with a bullet diameter of.286”. Woodleigh, out of Australia, makes a couple of flat based, pointed projectiles of .287” diameter which are available at Huntington Die Specialties. One person who knows a heck of a lot more about the subject of boring and rifling military rifles told me that the deeper grooves meant taller lands, which meant a longer serviceable bore life, as it’s primarily the lands which erode in use, not the bore. Which makes sense when you think about all the articles in which people have written about setting a barrel back and rechambering it to restore accuracy.
All the best - Dave
All the best - Dave
In that time period the bores [ lands ] of military rifles were kept close to spec but the grooves just had to be close and above minimum . They wanted the bullets to be smaller than the groove and to ride the bore . This gave the most velocity [ range ], less jacket fouling problems and kept pressures down . This started with the long and heavy for caliber bullets first used , they had a lot of bearing surface . Later when they went to shorter spitzer bullets they changed it up some , with countries trying different solutions - hollow base bullets , driving bands , smaller grooves , larger bullets . With the 7mm it was first a long rn .283 / .284 bullet in a .287 + groove . Some of the later spitzer ammo had bullets up to .286 . The Gew-88 .3188 bullet in a .3208 groove , US Krag .306 bullet in a .309 - .311 groove , .303 Brit a .309 bullet in a .312 up to .315 groove . The deeper grooves did also extend barrel life , that is why the Germans quickly went from a .314 - .3208 to a 311 - .3208 barrel , then to .311 - .323 + .