Which fill nipple cover is preferred to protect your equipment. There seems to be three (3) type 1 magnetic, 2 ballbearing, and plastic covers
Here are 4 types I have found on amazon for reference for any individual that don't know or don't have:
Custom Products (ballbearing) $10-13
http://www.amazon.com/Custom-Products-Fill-Nipple-Dust/dp/B0002WPRUY/ref=pd_sim_sbs_200_1?ie=UTF8&dpID=41ZnM8-d4CL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6
Omni (ballbearing) $6
http://www.amazon.com/Omni-Nipple-Cover-Paintball-Tanks/dp/B012B4PZMU/ref=sr_1_5?s=paintball&ie=UTF8&qid=1445957397&sr=1-5&keywords=fill+nipple+cover
Ninja (Magnetic) $10-13
http://www.amazon.com/Ninja-Paintball-Magnetic-Nipple-Cover/dp/B003FVP7O0/ref=pd_sim_sbs_200_3?ie=UTF8&dpID=31Bh62vgnGL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6
Rubber Nipple cover $5 for a pack10
http://www.amazon.com/Rubber-Fill-Nipple-Cover-Pack/dp/B00H94YQH8/ref=pd_sim_sbs_200_2?ie=UTF8&dpID=41pg48YW7VL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6
Here are 4 types I have found on amazon for reference for any individual that don't know or don't have:
Custom Products (ballbearing) $10-13
http://www.amazon.com/Custom-Products-Fill-Nipple-Dust/dp/B0002WPRUY/ref=pd_sim_sbs_200_1?ie=UTF8&dpID=41ZnM8-d4CL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6
Omni (ballbearing) $6
http://www.amazon.com/Omni-Nipple-Cover-Paintball-Tanks/dp/B012B4PZMU/ref=sr_1_5?s=paintball&ie=UTF8&qid=1445957397&sr=1-5&keywords=fill+nipple+cover
Ninja (Magnetic) $10-13
http://www.amazon.com/Ninja-Paintball-Magnetic-Nipple-Cover/dp/B003FVP7O0/ref=pd_sim_sbs_200_3?ie=UTF8&dpID=31Bh62vgnGL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6
Rubber Nipple cover $5 for a pack10
http://www.amazon.com/Rubber-Fill-Nipple-Cover-Pack/dp/B00H94YQH8/ref=pd_sim_sbs_200_2?ie=UTF8&dpID=41pg48YW7VL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1877K935K3V07KJJGCT6