Isn't the shooting jacket just a convenient place to attach your sling?
Personally, I really hope not, otherwise I have wasted a serious amount of cash over the years which could have been better spent on bullets or beer; however the questioner has actually got a bit of a point. What do jackets actually do for us and quite why do we spend so much money on them? First, an experiment...
|SCATT Practice in the garage. Hard to see from this angle, but the jacket is completely undone.|
If you're a regular reader, it cannot have escaped your attention that I like experimental evidence because it tells us the way the world really works, and not how people believe that it should work. In this case, I wanted to see what effect completely undoing my jacket would have on SCATT as some smallbore shooters are known to do in the prone position. As an experiment to see what effect this has on support and stability*, this was done completely on a whim so I don't pretend that it is statistically valid, but it is at least interesting.
While the group was slightly larger, the vital numbers are essentially no different, even taking account of the odd third shot in the second string, as the screen captures of the SCATT sessions below demonstrate.
|Ignore shot 3 in the second string. Probably interference.|
Qualitatively, the hold did not feel as secure without the jacket done up, and certainly when not on final aim it did seem to wave about quite a bit. Additionally, the butt of the rifle was harder to place in the shoulder and I don't feel that it would have behaved quite the same under recoil; however this is essentially conjecture without a trial. In conclusion, despite the results from SCATT being essentially indistinguishable I won't be shooting with my jacket undone any time soon!
So, what have shooting jackets ever done for us?
In addition to being an excellent place to attach your sling and make sure that it stays roughly in the same place relative to your musculo-skeletal system, your jacket fulfils a number of other functions in fullbore rifle shooting: it provides support and allows you to maintain the proper position for longer and more consistently as a result; the rubber on the elbows and in the shoulder prevent you from slipping between shots or while in the aim; the stiffness and thickness of the material helps soak up a little bit of the recoil, which reduces fatigue, and it helps to isolate the rifle from your heartbeat; and finally it gives you some protection from the elements that we have to endure when shooting outdoors.
It is possible to shoot decent scores without using a full shooting jacket; I remember forgetting my jacket many years ago, improvising with a greatcoat and a belt as slingkeeper and shooting a 35.5 at 300 yards; AP of Australia won the Corporation of the City of London match at 1000 yards with a score of 50.9 (dropping his last shot to a cooking bull, no less) in 2016 using a harness-like arrangement sold by a well-known UK shooting tailor. Despite this, I would suggest that your scores are going to be more consistent using a well-fitting jacket of roughly conventional design.
* Yes, before you all shout at me, I know that a lot of the stability for the sling position in particular comes from the fit across the shoulders and upper arms.