Hi Keltecfan,
I had a 6" Glock factory barrel in 10mm Auto, and hoped to fit it into a G41, so I purchased a G41. I had used this 6" barrel in my G20, Gen 3 for a few shots, but found it bulky, although it gave increased velocity over that of my G29 and G20. It also showed less muzzle flash when using BlueDOt in my reloads, and recoil felt less despite slightly higher velocity. However, the barrel diameter of the G20 barrel is 0.5mm larger in diameter than the G41 barrel, and does not fit through the slide opening at the front of the slide of the G41. The chamber section (rectangular in shape) is also 0.5mm wider in diameter and will not fit into the G41 slide.
Based on the above, I believe the G40 will be similar to the G41 in size, meaning a 5.3" barrel and not 6", and will be narrower through the slide than the G20.
Pity you missed the 6" Glock 10mm Auto barrel I sold here on GCN about a week ago.
RSA1