My friend has a red Boomco Halo MA5 blaster dart gun on which his kid accidentally broke the tip off. I took off the tip on my blue blaster and copied it in Fusion360 (with some minor overhangs changed to chamfers to avoid needing any supports) and printed it. Fits like a glove!
I printed this in PLA, although PETG or ABS may be better for durability vs. kids. I used 4 perimeters and 30% infill for strength.
I used a 5mm brim on some generic PLA with no supports and it printed just fine.