Accuracy + range= barrel + hop-up

[Terminator]

I was working on my M14 and was was looking to get better accuracy and range than stock for some time now. Well, after a lot of trial and error I have come to this conclusion.

"Accuracy and range" are direct results of the backspin (hop-up) and power output that your AEG provides. They have little to do with the barrel length directly. With that said, precision BBs and a tightbore barrel were used and certainly made a big difference. Accuracy was better but my distance wasn't what I was hoping for with this change. I figured that due to the fixed cylinder volume, as you increase the spring rate, the barrel actually has to become shorter to fully utilize the energy output. So depending on the spring rate you use, you may gain or lose muzzle energy by changing the barrel length.

The barrel length can influence your muzzle energy, and in turn may give you more or less range. From experiments, I know that cylinder to barrel volume ratio is not fixed. One thing that is certain is that the cylinder volume should always exceed the barrel volume**. With a stock spring, that ratio generally falls between 1.5:1 and 2:1. As higher rate springs are used, that ratio also has to increase to compensate for the increased air resistance on the BB. Heavier BBs will also benefit from the increased ratio because they require more pressure to move. If you cannot increase the cylinder volume (Bore-Up), you may need to decrease the barrel volume by shortening the barrel length. So if you have access to a shorter barrel, you should try measuring the muzzle energy with the different barrel lengths and see how it influences your setup's performance. So after changing my tightbore barrel to a shorter one, stronger spring and heavier BB I found that I managed to get both accuracy and range. 200ft shot within a 2in circle as 398fps.

The reason for requiring more cylinder volume over barrel volume is air compression. Most people have the misconception that cylinder volume should equate to the barrel volume. However, cylinder air does not move linearly out through the barrel like the denser liquids in a syringe. It is quite obvious from piston impact tests. With a stock spring in a PSG-1 mechbox assembly and a 110mm barrel (MP5K), the piston assembly still impacts the cylinder head before the BB leaves the barrel. The air is actually compressed until it can overcome the BB's weight, then the air expands and propagates down the barrel. So if cylinder volume only equates to the barrel volume, there would not be enough energy to accelerate the BB all the way down the barrel since the propelling pressure decreases as the air density equalizes. Well, now to get it to do the same with a 300 fps limit. The challanges never end.

[Righteous]

There seems to be a sweet spot for the bore/barrel volume ratio that will achieve the best velocity for a given weight bb and spring power. In other words how much barrel length will allow the bb to accelerate to the maximum the cylinder power will allow before frictional forces between the bb and barrel, gravity anf air resistance conspire to slow the bb down in the barrel. A longer tightbore barrel will still yield greater accuracy as the longer barrel will stabilise the bb better than a shorter one provided the barrel is not so long that it slows the bb down too much before it leaves the barrel.
The slug of air propelling the bb appears to reach its maximum velocity before the bb leaves the barrel, hence the bb will reach its maximum velocity before leaving the barrel or just as it does leave the barrel depending upon the amount of friction it encounters. This seems to be thre reason for the newer "highly polished" bbs being marketed towards snipers and competitions. There's a lot to take into account here. Also, how does porting the cylinder affect the power and accuracy?

[Terminator]

Polishing does help with sealing the piston head O-ring and cylinder head to the cylinder. I noted a 327-334 fps with five shots when I started. I went on a polishing frenzy using Braso and a drill. I polished the cylinder, cylinder head nozzle and inside/out, and nozzle. I used a little teflon to seal the head on. I then fired the same five shot group and it fired a CONSISTENT 333/334 pfs. I didn't note any significant grouping improvement in accuracy. It was similar to the initial shot group. Good for starting to fine tune though, which I felt was more important.

[hisoka]

Terminator what specific barrel, spring, cylinder,hop-up and BB exactly did you use? 2inch grouping @ 200' is quite impressive for an AEG.I'm just starting out in air soft tuning,so far i think i got most of the basics about good and consistent power,403-405fps/ 0.25g. But getting the accuracy has proved more challenging.I think cylinder to barrel volume matching is one of the shadiest part of air-soft tuning along with the hop-up.Needs a lot of trial and error and experimentation.