Receiver Shock Absorber Materials


The Franchi parts were (I think) made out of cast Cast Nylon 6, not a very good material.  It yellowed and became brittle over time.  The tit that held the part in finally broke off  on most of them, with the rest suffering a fracture of the whole part into shards.  Nylon is great for easily, cheaply molded parts, but they just don't last.   It has an impact strength of around 0.4 ft.-lbs./in. and hardness of around Rockwell R115.

It is not uncommon for me to hear about a brand new SPAS12 right out of the box to be fired and break the shock absorber with the first shot.

Ultra High Molecular Weight (UHMW) Polyethylene

Since I am machining each part, I can use a much better material.  The new parts are made of Ultra High Molecular Weight (UHMW) Polyethylene.  This material has about the highest impact strength of any plastic at any price -- and what we want is Impact Strength.

Impact Strength -- The ability of a material to withstand shock loading. Determined by the notched Izod test, which measures the effect on a
material when it is suddenly impacted by a swinging pendulum. A larger number signifies greater impact resistance.

Note almost four times the impact strength of "unbreakable" Polycarbonate.  One Hundred times the impact strength of Cast Nylon.  I offer a lifetime warranty against the part splitting or shattering.  You can take one of my shock absorbers and attack it with a 15-pound sledge hammer -- you will not damage it.

I am using a special type called Oil-Filled UHMW Polyethylene.   This has oil impregnated within the Polyethylene.  This actually doubles its tensile strength and lets it survive down to -450° F.  The oil helps to make anything contacting slides and does not stick and dig in.

This does not need the metal load spreading steel disk on the factory part.  The "old style" had no disk, but often broke up.  They added the disk to the new part to try and get their Nylon to survive.  The superior material I am using will not break up under any circumstances and needs no disk.   Also, the new part is held in by a stainless steel screw from the other side rather that the "tit" that always broke off.  The "tit" helped with quick mass assembly -- the screw will last much longer.

How About Polyurethane?

I have seen some parts being offered as of late made from Polyurethane.  They have issues with using a tit and a lack of expansion grooves, but let's just look at the material.

Polyurethane is not really a plastic, not really a rubber, half way in-between.  This makes many direct comparisons of physical properties difficult.  It does have the beginnings of making a  good shock absorber.  But it is not all that great a material in many respects.

Maximum Temperature

Note that Polyurethane is not rated for use above 150°F.

Oil Resistance

A material's ability to maintain its physical properties when it comes in contact with petroleum-based fluids and to resist absorption/swelling.

The UHMW Polyethylene I use is not only oil-resistant -- it's actually oil impregnated!  

Weather Resistance
How well a material withstands exposure to sunlight, oxygen, and ozone without breaking down.

Chemical Resistance
Indicates that a material can be used in a wide range of chemical environments while still retaining its physical properties.

Polyurethane parts will be bothered by oil, weather, and chemicals like gun cleaners over time.

One last note on Polyurethane.  Just like there are many types of Polyethylene (the best for our use being the UHMW), there are many types of Polyurethane.  I use 
Polyurethane in my folding stock shock absorbers -- and I use a thermoplastic type that is pushed through an extruder.  This is not to be confused with the two-part "epoxy" mixes that are used for home castings. These can have a wide range of hardness from batch to batch, and unless the maker is careful to use a vacuum chamber to degas the material, it will have tiny air bubbles in it.  Perhaps big enough to see, perhaps not, but not good to have in a part, and no where near the durability of the extruded thermoplastic material used by me (and automotive race suspensions).

Comments? Questions? Additions?  Corrections?