Did you remove the dart tooth? If so, then you're just pushing the stefan forward and not loading it into the breech.

What do you mean about the dart tooth? Where is it?

They fit in the barrel pretty loosely.

That, right there, is your problem. Also, washers tend to be heavier than most weights, so something like bbs or fishing weights are better for lower powered guns. But your main problem is still dart fit. Spring guns need a pretty tight fit.

what type of backer rod would recommend for a better fit?

I made some washer darts. They don't even shoot as far as a streamline out os a LS. The longshot is un modded. I also shot them out of a Firefly with the AR's taken out. They shot under 10 feet.

I put a hole in the back of the dart. Like stock ones.

I got backer rod from Lowes. I got a bunch of #8 washers and felt pads from there also. The darts I made seem to be noticeably heavier than the streamline darts. They fit in the barrel pretty loosely.

Should the washer darts perform as well as stock darts in an unmodded gun?
Should the washer darts be heavier than stock streamlines?
Does the dart need to fit tighter in the barrel?

Any help would be appreciated.