Hammer-Forged Accuracy

From Dave Sengelaub, Sunnen Products Company. Hammer-forged barrels are not well thought of by precision rifle shooters but a critical manufacturing process used by the hammer forging guys is suddenly getting attention from custom barrel makers today. It’s honing. Honing produces a high-quality bore surface fast, which is critical to hammer forging. Surface imperfections in a barrel blank before hammer forging will tend to be amplified as the blank is formed on the rifling mandrel.

And if the bore is chromed afterward, imperfections in the surface finish become even more obvious. Surface finish is the one feature of the barrel that cannot be controlled in hammer forging—garbage in garbage out. You may ask, "What about a reamed bore finish?" Sure, hammer-forged barrel blanks are gun drilled and reamed, just like all barrels, but a reamer starts to dull after the first part and scratches in the bore get rougher.

For this reason, makers of hammer-forged barrels regularly hone the bores of barrel blanks to enhance the product and improve process consistency. Honing dramatically improves bore diameter size uniformity and accuracy, surface finish, and roundness throughout the length of the barrel. It can certainly be used in place of a pre-rifling lap. The chief difference between a lapped and honed bore is the direction of the finish lines in the bore.

Honing leaves fine spiraling crosshatch lines, while a lap leaves lines going longitudinally in the bore. After rifling, the manufacturer can remove the crosshatch finish with a quick lap if desired. Honing is fast, accurate, and can be automated. Its surface quality and geometry can duplicate lapping, except for the longitudinal lines of the lapped finish. Honing is one of the processes used to make affordable, massproduced, hammer-forged barrels that regularly hit MOA (minute of angle) accuracy.

Mere MOA accuracy may be a joke to precision bench rest shooters but it’s a breakthrough for the consumer market and competition has some manufacturers bragging about half-MOA as their new standard. For custom barrel makers, honing is a time-saver and cost cutter. A few minutes on a honing machine can cut lapping times in half, leaving a crosshatched surface finish in single or low double-digit Ra.

Honing is the same process used to make diesel fuel injectors with bore roundness and straightness controlled to fractions of a micron (<0.000040"), with surface finish Ra ≤0.15 μm (6 μin). Sunnen Products Company (sunnen.com, makes these machines and has been a supplier to defense contractors and commercial firearms makers for decades. The company recently introduced anew machine for .17 to .50 caliber. Its spindle can correct bore size imperfections so small only an air gage can measure them.

Early users aim to make sub-minute accuracy the norm for their standard rifle barrels. Custom barrel makers are catching on to this, too, so it’s time for a tip of the hat to the hammer forging guys for leading the way.