Groove Diameter of 7x57 Mauser

[DB404]

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