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Normal Topic Does my firearm fit correctly - part three (Read 556 times)
50BMG
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Does my firearm fit correctly - part three
Aug 27th, 2018 at 3:01am
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There are five main contact points between the firer and the firearm. Each one has specific influences on ergonomics, control and the biomechanics of the aiming and firing process. Not really newsworthy.

These five points of contact are the handgrip, trigger, comb (cheekweld), Buttplate and support hand.  Additional mechanical influences are the type, height and location of sight, Fore-end support (bag, bipod, branch etc), Trigger weight and motion, Trigger shoe configuration, Weight and balance of the rifle, vertical centre of gravity (stability) of the rifle and a few other things. When using optical sights, eye relief, exit pupil dimensions as well as the need to be able to correctly access and manipulate turrets etc without upsetting the point of aim come into play.

What makes the fitting process less than straightforward is that the five main items identified above are all interrelated and can magnify or diminish the effect of each other in what is often seen as totally random ways. Making this even more confusing is the basic human form is utterly inconsistent.

When fitting a firearm some traditional terms are used such as length of Pull, drop at comb etc and are reasonably well understood. Not so well understood are the various human factors affecting fit. They are Interocular distance, centre of pupil to cheekbone at point of contact with comb, depth of soft tissue over cheekbone (join of Zygomatic and Maxillary bones), angle of cheekbone, shape and size of the Browbone (Frontal bone) and depth of soft tissue covering, neck length, distance from cheekbone to join of deltoid/ pectoral muscle (buttplate point of contact), position of Clavicle (collarbone) upper arm fore/ arm length ratio, Wrist bone alignment, Wrist comfortable range of motion and strength, Trigger finger length and muscularity/ tissue cover, trigger finger sensory/feedback depth of resolution, Trigger finger joint alignment, Support arm/ hand length, grip strength and comfortable range of motion, Fine motor control of each of the above.

So let's say that we have a firer with longish forearms, a short neck, reduced interocular distance, pronounced cheekbone, very lightly muscled frame (skinny) and pronounced clavicle. His measurements are 182cm tall, 87 kg,

Couple that to a “typical” rifle dimension which was set in the very late 1800’s to suit a “medium to larger an” at 5”8” to 5” 10” in height over average weight at 9 to 12 stone. Optics were not considered back then…. The net result is that most firearms produced today are produced to this same “standard”  and are perfectly suited (in theory) to a person in today’s language  - Male, 173 to 178 cm tall, 57 -76 kg, being of an athletic build using open sights with the aperture set at 20 mm above bore centre/axis. A drop at comb of 1 inch or 25mm is the standard.

Add a scope with a 40mm objective, 25mm ocular lens. Typically this will require a scope axis height to be set at least 23 mm above the top of the receiver to allow for rings and gain ocular bell clearance. This has completely destroyed the “Standard gun” dimensions to which virtually every firearm made today is based on. There is of recent times, a big move toward more and more complex stock/chassis systems provide compensatory adjustment, most of which originate in the military sniper system development.

It is easy to see that unless you are “the perfect human” as seen by designers of the late 1880’s, your firearm of choice simply will not be a proper fit straight out of the box. Unless, of course, it is a Holland and Holland or other similarly bespoke firearm made specifically for you by people qualified to measure you and make the firearm for you. (As a side note, I inherited 23 H&H firearms form my grandfather, all made specifically for him from the same reserved sample of Oak, and serial numbered accordingly. They range from .22 to 600Ne and 410, 16G and 12G. I have exactly the same target diagnoses for each and every one of the firearms. All indicate my arms need to be longer and my cheekbone to ocular distance less (decreased length of pull and reduced comb height). The errors are of identical measurement regardless of which Firearm I use) You cannot argue with this sort of evidence. The fit of the firearm really does matter and has a real, measurable and consistent effect on accuracy.

The drawback with getting a perfect fit is that a rifle that fits perfectly for the prone position over a bipod will not be very well fitting for a standing “offhand” shot. So you decide how you are going to use a particular firearm and then have it fitted for that purpose, knowing it will not work very well used differently. This is where properly adjustable Chassis systems come in. You can spend the time, take notes and then make adjustment for fit in the same way you adjust your turrets for drop and windage etc. It is the same basic principle of recording actual effect in a given circumstance and then adjusting for new known conditions.
« Last Edit: Aug 27th, 2018 at 9:06am by 50BMG »  

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