I think most of the resistance is inside the ammeter rather than the leads, anyway. I measured the resistance of my Extech ammeter with 6” 12 gauge leads at 0.035 ohms. A tailswitch with bypassed spring might have 0.01 ohms, but some switches might have more like 0.03 ohms.
A simple graphical tool I made can predict the resulting current with different ammeter resistances in a direct drive situation. Explained here and here.
For example, a bypassed tailswitch with 0.015ohms with a XPL might result in 6.04A.
graph 4.2-x**(.026.01.015) and (.19**(x/1)+2.74)
With a a 0.06 ohm ammeter instead of the tailcap, you would get about 5.1A.
graph 4.2-x**(.026.01.06) and (.19**(x/1)+2.74)
Using a clamp meter the tail resistance becomes negligible, ~0.001 ohms, and the resulting current would be about 6.44A.
graph 4.2-x**(.026.01.001) and (.19**(x/1)+2.74)
For a triple XPL the difference becomes relatively greater, 9.1A for the 0.06 ohm ammeter compared to 14.5A for the clamp meter.