If you plan on using these in traditional Pintle Sockets you will need an adapter. We do offer those adapters here: https://beltfeds.com/product/m134-pintle-adapters-for-use-with-dillon-type-vertical-arms/
These are milled from a solid block of 6061-T6511 Aluminum. Brass inserts are placed at the pintle locations (both top and bottom). The manufacture choose material type to achieve a light weight vertical arm with a high strength. 30,000psi yield / 42,000psi tensile.. 6061-T6511 aluminum is 6061 aluminum in the T6511 temper. To achieve this temper, the metal is solution heat-treated, stress relieved, then artificially aged. The stress relief is accomplished by stretching the metal by an amount that depends on the type of standard wrought product being made (extrusion or tube). The metal is straightened after the stretching operation.