Introducing the first of many design series posts by those at Porter Machine Works aimed at showcasing the expertise behind the development, industrial design and engineering of every piece of equipment purchased.
The theme of this post is tolerances, and why they rule the industry.
Perfect Parts Don’t Exist
Due to the variation caused by material characteristics and manufacturing processes, parts are never made to perfect specifications. They will always be made slightly larger or slightly smaller than their nominal design. The variation is captured in design as tolerances; the range of variation acceptable in the design.
Tolerances and Cost
The cost and performance of PMW rings is directly related to the tolerance analysis completed during the design stages. The tighter, smaller the tolerance the more difficult a part is to produce. And the firearms industry values the small numbers.
It is quite acceptable to find the standard use of 0.025mm (0.001”) on machined parts in the industry. What you’re looking at there is about a quarter of the thickness of a human hair.
A number of considerations are made when designing parts for manufacture. Many come down to the products requiring a tight and reliable fit to surfaces from varying manufacturers, of different materials and different coatings.
Let’s look at the cross section dimensions of the picatinny rail.
The NATO standard has three contact surfaces. These can range from 18.95mm to 19.05mm in width and 4.17mm to 3.92mm in height with any value falling within these ranges deemed acceptable.
One of the first questions is how to ensure that the rings fit correctly to the range of rails available to shooters everywhere. Up for consideration are the levels in quality available on the market as well as the different coatings that might be added; Cerakote can add 0.02mm to the overall thickness of a part.
See below an example. The same set of Porter Machine Works rings on rails with a different finishes; Cerakote (left) and anodising (right). The first clearly shows the gap on the ring’s clamp slot (despite Cerakote finish adding to the final dimensions of a finished part) where the second, anodised rail has minimal gap.
Prototyping is our friend, and to make sure that the Porter Machine Works rings are always the best foundation for your optic setup, each extreme case is considered and explored during the prototyping stages. Coupled with adopting the best industry standards and practices in design and machining, you the shooter can be certain that a set of rings from Porter Machine Works will be the best platform for whatever kit you’re running, for whatever purpose.
It goes without saying, choosing the right rifle and scope for the task at hand is critical to a successful day’s shooting. The difference between a gratifying hit and a taunting miss however, can and will come down to your foundations; A quality, comfortable and well-made stock, the small but all important scope rings and providing ease of aiming, the lightest rear bag locally available.
The best thing about this combination is that it’s all available from Australian companies.
It all started with the Lithgow Arms LA102 chambered in .243 Winchester dropped into the
Bolly UPH Gen3 stock.
Until now, Lithgow LA102 .243 had always given a bit of a curve ball, namely the height of the picatinny rail from the bore axis. But on this day, that wasn’t the case and the Porter Machine Works 34mm-22 low rings paired with the Delta Optical Stryker HD 5-50x56 was the perfect setup to get the ideal cheekweld.
The weather wasn’t ideal with some rain and gusty wind but there was some determination to get the rifle zeroed, so the setup included a tarp cover and carpet floor, and with the Low Vis Gear rear bag, Atlas bipod and 87Gr Buffalo River ammunition , it was all ready to go.
The Lithgow LA102 .243 was bore sighted at 100m and it was touch and go with the wind gusts. First shot was way high and way left, and second shot landed an inch right of the first.
In a short, calm moment a rushed adjustment led to managing a 5 shot group. Considering the conditions, the system was working against, it couldn’t be blamed for the slightly vertical but clearly sub MOA that had been dialed in a rush.
Now it was time for more precise adjustments for a 100m zero; the SFP scope was set to the right magnification for true subtensions.
The bullseye first round hit, with the second and third appearing as a clover leaf. A combination of the 3 round limit in the magazine and the temperamental conditions spoiled the fun and sending the last 3 shots down couldn’t have come soon enough to finish the group which had moved .25moa up and left.
But, looking back at us, despite the conditions was a 0.4moa group and a very happy shooter.
Scope ring choices are worthy of an entire article, but here we see how important each component of the system is when it comes to accurate results. Your rings hold your sighting system in relation to the bore of the rifle and without this being perfectly consistent you will never fully realise the accuracy the rifle is capable of. This is especially the case for the Lithgow .243 as the Lithgow's are known for producing beautiful results straight out of the box. Without careful consideration in pairing the right equipment together, managing to find perfect conditions will not be your saviour.
With the huge number of combinations on the market it is very easy to get rings that are not the right match for your scope and rifle. Here you have seen the right setup will give you the opportunity to measure and improve on your technique and ability, and you’ll be free to tackle any scenario.
Thank you to Precision Defence Industries for the setup, Lithgow Arms for making a great .243 barrel and LowVis Gear for an awesome rear bag.
If you would like to know more about how to get the best out of your setup and your scope rings, let us know.