I am former military (Army Reserve) and when I was in basic training we had a couple of weeks of range time and classes to become proficient with the M16 A2 rifle. We were taught breath control, trigger discipline, sight alignment, sight picture, and rear and front sight tweaking to bring the bullet on target effectively.
I was in the reserves for 8 years total and we would go to the range about once a year to re-qualify and I never had a problem with it. When I was in the process of deploying to Iraq in '03 I was assigned a weapon that was totally different from the M16. I was issued an M249 SAW.
Went to the range two different days to sight in, become proficient, and qualify with the weapon. I did extremely well with it (best in the brigade as a matter of fact) and ended up trusting my life to that weapon while stationed in Baghdad on the material collection teams. It always hit what I was aiming at, but again my training was mainly in a different weapon system.
So as my forum mates above said, becoming proficient with shooting a caliber that is more efficient economically is a really good idea. The fact that I didn't buy my ammo in the army helped a lot in becoming proficient with the 16 and the 249. I wouldn't be anywhere near as good if I had to pay for it. (Simply because I wouldn't have been able to afford it in the first place
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The round I became proficient with in the 16 was the same exact round that was fired out of the 249 just at a different rate and style of fire. The 16 was semi-automatic or 3 round burst and the 249 was fully automatic. Being able to plug an M16 clip into the 249 in emergencies was a definate plus. Doing so kind of chewed up the M16 magazine some however.
While I was overseas, I hit the range about once a week with my 249 and captured or otherwise acquired ammunition. I maintained proficiency with my 249 for the entire duration of the deployment. And, I wager, if i were to pick one up and give it a whirl today (been out of the Army since 09) I would probably do fairly well with it.
But now that I am out of the Army I still use the same principles and disciplines that were taught to me during "range week" in training. (They called it range week even though it was actually 2 weeks)
Breath control, trigger discipline, sight picture, proper prone unsupported and supported firing, maintaining a good cheek weld, and keeping the rifle tucked tightly away in your "shoulder pocket" were all attributes to everyones success at the range.
There are lots of veterans in TFL forums and they will all tell you that once you know what to do with a weapon, and learn the simple disciplines involved in firing, the rest just seems to fall into place. And the targets will also fall.
The quaility of your instruction, the quality of your equipment and how seriously you take your practice all will be factors.
+1 ndking1126
I have never hunted anything other than 2 legged varmints so you need to rely on the advice of my fellow TFL members on that one. I really want to get into some hunting (4 legged animals) one day but I need to train up on the use of optics for that. Never shot something through a scope before. And I know you don't have to (use a scope), but most hunters I see these days use some kind of glass.
To answer the question of the OP, it really depends on the quality of training and your willingness to learn and apply the training, but for me, it took about 2 weeks.