Green Gas vs HPA on the M870
A stock M870 runs on green gas stored in an onboard reservoir. This is simple and untethered, but FPS drops as the gun cools during rapid firing, and cold weather reduces both power and shot count. HPA feeds the gun regulated air from an external tank through a line, so pressure stays constant. The result is stable FPS shot after shot, no cool-down, reliable cold-weather performance, and the ability to dial FPS to your field limit. The tradeoff is that you are now connected to a tank by a line, which changes how you carry and run the gun.
HPA Conversion Components
An M870 HPA conversion is built from a few core parts. This table lists what each part does and its approximate cost.
| Component | Approx. Cost | Purpose |
|---|---|---|
| HPA air tank (aluminum) | $50 to $70 | Stores regulated high pressure air |
| Regulator + line combo | $100 to $160 | Sets and holds working pressure; brands include Redline, Wolverine, and PolarStar |
| Tap / fittings (e.g. Balystik Mag Tap) | $15 to $25 | Feeds air into the gun on removable-stock models |
| Internal Gas Line (IGL) | Varies | Alternative feed method for fixed-stock guns; routes air internally |
Total Cost of an M870 HPA Conversion
A complete M870 HPA conversion typically costs about $165 to $255 or more, depending on the tank, regulator brand, and fitting method you choose. A budget build using an aluminum tank, a mid-tier regulator, and a simple tap sits near the lower end. A premium regulator from a brand like Redline, Wolverine, or PolarStar pushes the total higher. Choosing between a tap and an Internal Gas Line depends on whether your stock is removable or fixed.
What HPA Changes About Your Upgrade Plan
Converting to HPA makes some green gas upgrades irrelevant while leaving others just as important:
- •Obsolete after HPA: green gas O-ring seal sets and the integrated-reservoir stock are no longer needed, since the gun no longer stores its own gas.
- •Still critical: heavier BBs (0.28g to 0.32g) remain the most important tuning change for range and accuracy.
- •Still valuable: a tight-bore inner barrel and internal reinforcement parts (Wii-Tech and similar) still help, exactly as they do on a green gas gun.
- •Still avoid: aftermarket hop-up chambers and loading nozzles remain a common cause of feeding failures, HPA or not. Keep the stock loading system.
Pros and Cons of M870 HPA
Weigh these points before converting:
| Pros | Cons |
|---|---|
| Consistent FPS shot after shot (e.g. stable 330 FPS on 0.20g at 95 PSI) | Tethered to an external tank by a line |
| No cool-down during rapid fire | Higher upfront cost than a green gas tune |
| Reliable in cold weather | More setup and dialing-in required |
| FPS is fully adjustable to field limits | Less convenient to carry and move with |
When Should You Convert to HPA?
HPA makes the most sense if you want tournament-consistent FPS, play year-round including in the cold, or want to fine-tune power precisely to a field limit. If you value a simple, untethered gun and mostly play in mild weather, a green gas tune with heavier BBs and fresh seals may be all you need. Many players start with the green gas upgrades first and only convert to HPA once they want maximum consistency.
Frequently Asked Questions
Is HPA better than green gas for an airsoft M870?
HPA is better for consistency. It delivers stable FPS with no cool-down, works reliably in cold weather, and lets you adjust power precisely. Green gas is simpler and untethered but loses FPS as the gun cools and in cold conditions. HPA is the better choice if you prioritize consistent performance over convenience.
How much does it cost to convert an M870 to HPA?
A full M870 HPA conversion typically costs about $165 to $255 or more. This includes an aluminum air tank (roughly $50 to $70), a regulator and line combo ($100 to $160), and a tap or fittings ($15 to $25). Premium regulators from brands like Redline, Wolverine, or PolarStar raise the total.
Do I still need heavier BBs after converting my M870 to HPA?
Yes. Heavier BBs in the 0.28g to 0.32g range remain the most important tuning change for range and accuracy on the M870, whether it runs on green gas or HPA. Converting to HPA changes the power source, not the hop-up behavior.
What do I need to convert an M870 to HPA?
You need an HPA air tank, a regulator with a line, and a way to feed air into the gun. On removable-stock models this is usually a tap such as the Balystik Mag Tap. On fixed-stock models an Internal Gas Line routes the air internally instead. Regulator options include Redline, Wolverine, and PolarStar.
Does an HPA M870 still have feeding issues with aftermarket nozzles?
Yes. Aftermarket hop-up chambers and loading nozzles remain a common cause of feeding failures on the M870 regardless of the power source. Keep the stock loading system even after an HPA conversion unless a trusted tech recommends otherwise for your model.